A simple measure, usually related to cash flow, of the time taken to recover any investment or spend in terms of years and months. E.g. Day 1 Spend $100, Day 30 Receive $50, Day 60 Receive $50, Day 90 Receive $50, Payback = 60 days because that is the point in time at which the original $100 spend was recovered.
The 'Mickey Mouse' of all measures - but a real killer. It's the most basic question possible - 'If I give you $100 when will I finally get it all back'. It's a killer measure because it's the period of time during which you are without access to monies that are currently yours. This measure does not take any account of relative risk, ROI, interest rates or anything else that key decisions usually require.