An operational definition is a behavioral description of a behavior or action such that anyone else would be able to see the same behavior and judge it to be similar. For example, if we say that rats being used in an experiment were hungry, we would operationally define hunger as twenty-five percent below their starting weight. We might further explain that they were maintained at a certain weight by allowing them to eat for a certain amount of time each dat or a certain amount of food.
With people, if we wrote a program to reduce head slapping, we might want to operationally define head-slapping as "hitting head with open hand" as opposed to punching (hitting head with fist, closed hand) or hitting head against wall (self explanatory).