Involving three sides — three countries, three companies, three groups sitting at the same table. A trilateral meeting is one where all three are there together, each speaking for itself. That is the whole point of the word: not three separate conversations, but one conversation with three sides in it.

It lives almost entirely in news and government language — trilateral talks, a trilateral agreement, trilateral cooperation. You will meet it constantly in reporting about Taiwan, Japan, the US, Korea and China, because so much diplomacy happens in threes. Compare bilateral (two sides), which is far more common, and multilateral (many sides). Journalists sometimes shorten the pattern into a name: the Trilateral Commission is a real organisation, so a capital T can mean that specific group rather than the plain adjective.

In geometry it can simply mean 'having three sides', but that use is rare — for shapes people say triangular.

It is formal. In ordinary speech you would just say 'a meeting between the three countries'.