Definition:
The additive-increase multiplicative-decrease (AIMD) algorithm is a feedback control algorithm best known for its use in TCP congestion control. AIMD combines linear growth of the congestion window with an exponential reduction when congestion is detected. Multiple flows using AIMD congestion control will eventually converge to use equal amounts of a shared link. The related schemes of multiplicative-increase/multiplicative-decrease (MIMD) and additive-increase/additive-decrease (AIAD) do not reach stability.
Further Reading:
Book: Playing to win by A. G. Lafley and Roger L. Martin