When I'm referring to over population, I'm usually meaning of humans.
Over population is when an environment in an area does not have enough resources for the life form in question.
It is important to note this is not limited to what is needed for survival. There must be enough resources so everyone has everything they want.
And for sustainability, this means resources must not be depleted, damaged, polluted, or rendered unusable. Unless a situation exists where all resources are naturally sufficiently replenished, this would be the determining factor in how long before over population is reached and how quickly it becomes a problem.
Further, without intervention. Changes in the environment to produce more of something for example is an indication of over population.
Over population causes a range of emotions and problems, most significantly is theft and violence.
Easiest to imagine I think is to consider the complete opposite of over population. space food...
Unless brought under control in time, eventual extinction is the result. If not by killing each other off in pursuing usable resources, then by eventual depletion of usable resources and possibly deaths due to using unusable (too badly polluted) resources.