Gemini and other generative AI models process input and output at a granularity called a token.
About tokens
Tokens can be single characters like z or whole words like cat. Long words
are broken up into several tokens. The set of all tokens used by the model is
called the vocabulary, and the process of splitting text into tokens is called
tokenization.
For Gemini models, a token is equivalent to about 4 characters. 100 tokens is equal to about 60-80 English words.
When billing is enabled, the cost of a call to the Gemini API is determined in part by the number of input and output tokens, so knowing how to count tokens can be helpful.