Although there is no specific definition for micro I would define it as the following:
Maximal killing of enemy units with minimal losses of your own units through the use of strategical movement and optimal use of your units' advantages.
Micro can be used to survive when being outnumbered but it is generally used by skilled people all the time during gameplay.
Next to micro, a player also has his macro
to watch out for, these should be harmoniously balanced.