Science

Glossary

Short definitions of the terms that appear in our Science articles. Each one links to the sources we used.

Sources checked October 2026

Educational content, not medical advice. It explains general science and does not replace a conversation with a qualified clinician about your own health, tests or treatment.

Imaging

Attenuation: How much an X-ray beam is weakened as it passes through the body. Dense bone absorbs much of the beam, while soft tissue lets more of it through.; Source: RadiologyInfo.org (RSNA & ACR) (opens in a new tab)
Computed tomography(CT): An X-ray technique in which a source and detectors rotate around the patient so that a computer can build cross-sectional images (“slices”).; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Contrast agent: A substance given to make certain tissues or vessels stand out: iodine (into a vein) or barium (by mouth) for CT, and gadolinium for MRI.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab); National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
DICOM(Digital Imaging and Communications in Medicine): The international standard for medical images and related information, first published in 1993 and recognised by ISO as ISO 12052.; Source: DICOM Standards Committee (NEMA) (opens in a new tab)
Doppler ultrasound: An ultrasound technique for measuring and visualising blood flow in vessels and in the heart.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Double reading: Two readers interpret the same images independently, with consensus or arbitration when they disagree. It is widely used in European breast screening.; Source: European Commission, Joint Research Centre (opens in a new tab); British Journal of Radiology (opens in a new tab)
Hounsfield unit(HU): The scale for CT values. By definition water is 0 HU and air is −1000 HU; bone ranges from several hundred to several thousand HU.; Source: AMIA Annual Symposium Proceedings (opens in a new tab)
Ionizing radiation: Radiation with enough energy to harm living tissue. X-ray imaging and CT use it; MRI and ultrasound do not.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab); National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab); National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Magnetic resonance imaging(MRI): Imaging that uses a strong magnet and radiofrequency pulses to detect signals from protons in the body. It does not use ionizing radiation.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Millisievert(mSv): The unit commonly used for effective radiation dose from imaging, which lets the doses of different exams be compared with each other and with natural background radiation.; Source: RadiologyInfo.org (RSNA & ACR) (opens in a new tab)
Radiograph(X-ray image): An image made when X-rays that pass through the body reach a detector on the other side, recording the “shadows” of the structures they crossed.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Satisfaction of search(subsequent search miss): A drop in the chance of noticing a further abnormality once a first one has been found in the same image.; Source: Cognitive Research: Principles and Implications (opens in a new tab)
Transducer: The handheld ultrasound probe. Its piezoelectric crystals turn electrical signals into sound waves and returning echoes back into signals.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Ultrasound: Imaging with sound waves above the range of human hearing. A transducer sends pulses into the body and listens for the echoes that return from tissue boundaries.; Source: National Institute of Biomedical Imaging and Bioengineering (NIH) (opens in a new tab)
Window level(WL, window centre): The HU value at the middle of the window, shown as mid-grey.; Source: AMIA Annual Symposium Proceedings (opens in a new tab)
Window width(WW): The span of HU values shown as shades of grey. A wide window shows many tissue types at once; a narrow one exaggerates small differences.; Source: AMIA Annual Symposium Proceedings (opens in a new tab)
Windowing: Choosing which range of HU values to spread across the display’s grey levels. Values above the window show as white, values below as black.; Source: AMIA Annual Symposium Proceedings (opens in a new tab)

Genomics

Allele: One of the slightly different forms of the same gene. Each person inherits two copies of each gene, one from each parent.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Base pair: Two bases on opposite DNA strands joined together. A pairs with T, and C pairs with G.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Chromosome: A thread-like package of DNA. People usually have 23 pairs (46 in total): 22 pairs of autosomes and one pair of sex chromosomes.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Coverage(sequencing depth): How many reads overlap a base. Average depth is the number of reads × read length ÷ the size of the target; breadth is the share of the target covered at least a given number of times.; Source: Nature Reviews Genetics 15 (opens in a new tab)
DNA(deoxyribonucleic acid): The hereditary material in humans and almost all other organisms, written in four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T).; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Exome: All of the exons in the genome considered together.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Exon: The protein-coding parts of a gene. Together they are thought to make up about 1% of the genome.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Gene: A stretch of DNA, from a few hundred to more than two million bases long, that carries instructions. Most genes code for proteins; humans have about 19,900 protein-coding genes.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Gene panel: A test that looks at a selected set of genes, usually chosen for a particular condition or question.; Source: Genetics in Medicine (opens in a new tab)
Genome: The complete set of DNA in an organism. The human genome has about 3 billion bases.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab); National Human Genome Research Institute (NIH) (opens in a new tab)
Germline variant: A variant inherited from a parent through the egg or sperm. It is present in virtually every cell of the body.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Karyotype: A picture of a person’s chromosomes, arranged in pairs by size.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Next-generation sequencing(NGS): Newer, faster sequencing approaches that make it feasible to sequence large amounts of DNA, such as a whole exome or genome, in days to weeks.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Nucleotide: One building block of DNA: a base attached to a sugar molecule and a phosphate molecule.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Nutrigenomics: Research into how genetic make-up relates to diet, and how nutrition influences gene activity.; Source: OMICS: A Journal of Integrative Biology (opens in a new tab)
Pharmacogenomics: The study of how a person’s genes affect their response to drugs.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Read: A short stretch of DNA sequence produced by a sequencing machine. Reads are lined up against a reference sequence to find differences.; Source: Nature Reviews Genetics 15 (opens in a new tab)
Sanger sequencing: An earlier sequencing method, used by the Human Genome Project and still used for short pieces of DNA.; Source: National Human Genome Research Institute (NIH) (opens in a new tab); MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Single nucleotide polymorphism(SNP): A genomic variant at a single base position in the DNA, where one person’s sequence differs from another’s.; Source: National Human Genome Research Institute (NIH) (opens in a new tab)
Somatic variant: A variant acquired during a person’s life in certain cells only, for example from copying errors or UV exposure. It is not passed to children.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Variant: A permanent change in a DNA sequence. Most variants do not cause disease. Guidelines prefer this neutral word to “mutation” or “polymorphism”.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab); Genetics in Medicine (opens in a new tab)
Variant calling: Finding positions where the reads consistently differ from the reference sequence.; Source: Nature Reviews Genetics 15 (opens in a new tab)
Variant of uncertain significance(VUS): A variant without enough evidence to call it pathogenic or benign. Under ACMG/AMP guidance, a VUS should not be used in clinical decision-making.; Source: Genetics in Medicine (opens in a new tab)
Whole exome sequencing(WES): Sequencing only the exons. It can miss variants that lie outside them.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)
Whole genome sequencing(WGS): Sequencing that determines the order of all the nucleotides in a person’s DNA, both inside and outside genes.; Source: MedlinePlus Genetics (U.S. National Library of Medicine) (opens in a new tab)