What are key performance indicators (KPI) in marketing?

A KPI is used in marketing for determining the progress and growth toward the business goals.
A KPI in marketing is a measurable value tied to specific objectives of a marketing campaign. It indicates progress during the campaign and helps measure marketing effectiveness at the end of a campaign.
It is a very important tool which helps the organisation in achieving and measuring short terms goals such as measuring the effectiveness of ad campaigns or to measure the effectiveness of new marketing plan.