A velocity limit is a restriction on how often, or for how much in total, a card or account can be used within a defined window of time. An issuer might allow no more than ten purchases a day on a card, or cash withdrawals of no more than a fixed sum in 24 hours. When a new transaction would exceed the limit, it is declined during authorization.
Velocity limits are one of the standard spend controls in card issuing programs. They differ from a simple spending limit on a single purchase because they look at cumulative activity. Their main purpose is to contain losses when a card is stolen or its details are being tested by fraudsters. The same idea applied on the fraud detection side, across many cards or devices, is called a velocity check. Legitimate bursts of spending can trigger a payment decline, so limits need to match expected use.