Velocity check
A fraud rule that counts how often something happens within a time window and raises the alarm when it happens too fast.
The idea is simple: the same card number, IP address, email or device may only appear so many times per minute, hour or day. Twenty declined card attempts from one IP in a minute is not a customer with clumsy fingers — it is someone working through a stolen card list.
Velocity checks are the oldest form of automated fraud prevention and still sit at the bottom of almost every risk engine, underneath the machine-learning models. They are cheap, easy to explain to an auditor and easy to dodge for anyone who spreads the attempts across a thousand IP addresses. That is why modern rules count across several dimensions at once.