In numeral prefixes, the prefix “tri-” means three.
English uses tri- to signal a quantity or arrangement involving three parts, objects, or repetitions. Familiar examples include triangle, a shape with three sides; tricycle, a vehicle with three wheels; and triad, a group of three. In scientific and technical vocabulary, the prefix can describe a three-part structure, as in trilingual for someone using three languages or trimester for one of three divisions of a period.
Tri- is chiefly associated with Latin-derived forms, but it also appears in words influenced by Greek. The related Greek prefix is often written tri- as well, as in triathlon and tripod. A tripod has three legs, while a triathlon combines three athletic disciplines: swimming, cycling, and running.
The prefix is easy to confuse with bi-, meaning two, and quadri- or tetra-, meaning four. It also differs from ter-, a related Latin form used in words such as tertiary and ternary. In some compounds, tri- retains its spelling even before a vowel, unlike certain other numeral prefixes that change form. Recognizing tri- can help readers infer the meaning of unfamiliar words, although word origins and modern usage do not always follow a perfectly regular pattern.