Token
The smallest unit an LLM handles — roughly a word, syllable or character.
"Hello world" is 2 tokens. A compound word like "internationalisation" is often 5–6 tokens (longer words are split). Models charge per token; a price like $3/M input tokens means 3 dollars per million tokens you send in.
A model's context window is measured in tokens — e.g. 200,000 tokens is roughly 150,000 words (typically a short book). Anything above that limit won't fit in a single call.