An operational definition means defining the variable as it exists in the present study. For instance, memory is an incredibly broad term so any study that investigates memory must make a precise definition of how “memory” will be measured in that particular study. For an operationally defined DV the variable needs to be quantified.
So in an experiment on memory, for instance, memory might be quantified as “how many words participants can recall”, or “how many items the participants fail to recall”. Another study operationally defined aggression as “the amount of hot sauce a participant was willing to pay to have someone else consume”.