What is Michael addition?

A Michael addition is the conjugate addition of a stabilised carbanion, usually an enolate, to an $\alpha,\beta$-unsaturated carbonyl compound or a similar electron-poor alkene. The nucleophile is the Michael donor (a 1,3-dicarbonyl compound, a nitroalkane, a cyanoester); the unsaturated compound is the Michael acceptor (an enone, an enal, an unsaturated ester, nitrile or nitro compound).