Language models
The most text, counted in tokens, that a language model can take in and keep in view at one time, including its own answer.
Formal
The fixed limit on how many tokens a language model can handle in one go; the prompt, earlier turns of the chat, added documents and the answer must all fit inside it.
In plain English
Like the size of a desk - everything the model is working on has to fit on it, and when it is full, something has to be cleared away to make room.
In practice
A clerk in a region works with an AI assistant through a long afternoon; it starts ignoring the format rules given at the start, because the chat app has cut the oldest messages to keep the rest inside the context window.
Why it matters
It limits how much a model can weigh in one answer, and everything inside it - including text from a file nobody checked - shapes that answer.