Key Performance Indicator refers to the metrics or measures of a process that are indicative of overall performance.
Companies that measure performance should have set targets and standards for measuring performance on those things they consider to be really important. These measures are called Key Performance Indicators (KPIs). KPI’s measure factors that management believes are an indication of operational excellence. To be a realistic indicator, each KPI should be based on a reasonable target and should change over time as the business improves.
Source: Guide to the Business Process Management Body of Knowledge – ABPMP BPM CBOK V3.0.