Gene
A gene is a stretch of DNA that carries the instructions for a product, usually a protein or a functional RNA. Wilhelm Johannsen introduced the word in 1909 for a unit of heredity whose physical nature was then unknown; the molecular definition arrived decades later. The word is still used in two ways that do not fully line up: the classical gene, inferred from patterns of inheritance, and the molecular gene, a sequence with a start and an end, and the mismatch between them is an active dispute.
Why it matters
It is the single most reused word in biology, and knowing that it names two different things prevents most of the confusion beginners have with genetics papers.
Also written: genes, genic
