Token
The unit a language model reads and writes, and the unit almost all of them are billed by: roughly three-quarters of an English word.
what you are billed for · the vocabulary
tokenstokenizer
A model does not see letters or words. Text is cut into tokens — common words are one, rarer ones are several, and a space usually rides along with the word after it. Prices are quoted per million tokens, separately for what you send and what comes back, because generating is far more expensive than reading. As a working figure: 750 English words is about 1,000 tokens, and code and non-Latin scripts run higher.