"Fat finger" (also known as fat finger error) is a term that refers to input errors made when manually entering data into computers, via keyboards or touch screens. These errors are especially common in dynamic and stressful environments, such as trading floors and dealing rooms, and can sometimes have major financial consequences.

A fat finger error occurs when, for example, a trader accidentally enters the wrong amount or number, causing huge orders to be placed. This can lead to large price fluctuations, so-called "flash crashes," and chain reactions in financial markets, especially by computerized trading systems that react instantly to price and volume anomalies. It is a nightmare for any stock market trader and has already led to significant losses in the past.