What a “token” is, and why it's on your invoice.

You see “per 1,000 tokens” on an AI invoice and wonder what you're actually buying. A token is the unit underneath almost every AI price, every limit, and every “it forgot what I said.” Here it is in two minutes.

Models don't read words or letters — they read tokens: short chunks of text, roughly ¾ of a word. Common words are usually one token; longer or unusual words split into several. Your text is broken into tokens on the way in, and the model writes tokens on the way out.

• •It's on your invoice because tokens are the meter. Pricing is per token, in and out¹ — so a long document or a chatty back-and-forth costs more.
• •It's why AI “forgets.” The context window is a token budget for how much the model can hold at once; past that limit, earlier text falls out of view.
• •Longer isn't free. Pasting a whole report into every prompt is convenient and quietly expensive — one more reason retrieval (RAG) beats stuffing everything in. (Curious how text becomes tokens? There's a live tokenizer² you can paste into.)

The “so what”

When you scope an AI feature for a client, scope it in tokens: how big is the input, how long is the output, how often does it run? That's the number that drives the bill — and knowing it is the difference between a margin and a surprise.