What is multivibrator in electronics circuit?

Its an electronic circuit to implement two state systems such as oscillators and timers etc consisting of amplifying devices usually transistors which are cross coupled by resistor or capacitors.

Basically it is an oscillator using two transistors, resistors and capacitors  to generate a square wave signal in the kiloHertz frequency range. It is a chief alternative instead of using a555 Timer IC to produce an oscillation.