termino

A termino is not just any word. Esperanto separates three things English lets you call a "word" without blinking. Vorto is the plain unit of language, any word at all. Esprimo is an expression, a way of putting something, which may run to several words. A termino is narrower and sharper than both: it is a word with an agreed technical meaning inside a particular field, fixed by definition rather than by usage. Akvo is a vorto. Ebenaĵo in a geography paper is a termino. The difference is not size or difficulty, it is whether somebody has drawn a boundary round the meaning and written it down.

Which is exactly what the Latin says. Terminus was a boundary stone, the marker at the edge of a field, and Roman law took it into the sense of a limit or a defined end. Scholarly Latin then borrowed it for a word whose meaning has been fenced. English determine is the same picture: to determine something is to put a boundary round it. When you difini a termino, you are doing in Esperanto what the etymology has been describing all along, and this is one of the rare occasions where knowing the root actually helps you use the word.

Esperanto has an unusual stake in this. A language designed for international use lives or dies on whether its technical vocabulary is stable, and the movement has taken terminology seriously since the beginning: there is a body of subject dictionaries, an Akademio committee on it, and a long argument about whether the language should borrow international terminoj whole (civilizacio, komputilo versus kompjutero) or build them from its own roots. That argument is not settled and probably should not be. The article on Esperanto in science takes it up in detail.

Using it. Terminologio is terminology, the set of terms in a field. Termina is the adjective. Watch two neighbours: termo is heat, as in termometro, and terminalo is the transport or computing terminal, so termino, termo and terminalo are three different roots that a hurried reader will run together. And do not use termino where you mean a deadline: that is limdato, and the false friend is the English term in the sense of a fixed period, which Esperanto does not extend the word to cover.