Rate limit
A cap on how much you may send in a window, usually counted in requests and tokens per minute.
Also written: rate limits, TPM, RPM, quota.
Vendors cap throughput per account, usually as requests per minute and tokens per minute together. Exceeding either returns an error rather than a bigger bill.
This is the constraint that most often turns a working prototype into a broken product, and it is invisible in any price comparison. Before committing to a model, check the limits on the tier you will actually be on, not the limits on the page.