Token
The unit an LLM reads and writes - roughly three quarters of a word in English.
Models don't see letters or words, they see tokens. Pricing, rate limits and context windows are all measured in tokens, so a rough conversion is useful: 1,000 tokens ≈ 750 words ≈ 1.5 pages.
If a workflow feels expensive or slow, count tokens before blaming the model: pasting a 60-page PDF into every request is usually the real cost driver.