The special words that belong to one subject, job, or activity. Every field has its own set: doctors say 'benign' and 'lesion', lawyers say 'plaintiff' and 'tort', computer people say 'cache' and 'thread'. When you say 'medical terminology' or 'legal terminology', you mean the whole collection of those words taken together — not one word, but the vocabulary of that world.

The word is usually uncountable and often has 'of' or an adjective in front: 'the terminology of statistics', 'business terminology', 'technical terminology'. You can also use it in the plural ('terminologies') when you are comparing two different systems side by side, but that is rare and quite academic.

It carries a small feeling. When people say 'the terminology is confusing' or 'I don't know the terminology yet', they mean the words are a wall standing between them and the idea behind it. So the word often appears when someone is being shut out, or is admitting they are new. Teachers and writers also use it in the other direction — 'let me explain the terminology first' — meaning: before we can talk, I have to hand you the words.

It is a formal word, and it usually points at hard or technical words, not at slang or the special words a family or a group of friends share. For that you would say 'jargon' (which sounds unfriendly and shuts people out) or just 'the words we use'. 'Terminology' is the neutral, respectful one: it says these words are precise and exist for a reason, even if you don't know them yet.