Being depressed is not a vague concept, people's understanding may be vague but as far as the condition is concerned you either suffer from depression or you don't. If you have depression you may suffer infrequently or often. If you have depression you may suffer very severely or not as severely. The duration of your episodes may vary as well. They may last minutes, hours, weeks, months or even years. But regardless of how frequent, severe and long the episodes are you either have depression or you don't.
Feeling bad and having depression are two separate phenomena. For a depressive, feeling bad can trip the switch that may result in a depressive episode but they are still two distinct and separate phenomena where the depression acts as catalyst which greatly intensifies the bad feelings and makes it much harder to exit the mood. On the surface, the difference lies in how quickly and easily one can end the episode. Bad feelings and dour moods are normal and occur with differing frequencies, severities and durations for most individuals. These are all a function of many variables including temperament, circumstances, and various other aspects regarding the individual. But whether or not these lead to a state of depression is another issue entirely. Feeling bad is like stopping your car, you should be able to restart it and move on if you wish. But feeling bad when depression is involved means your car is stopped and it is very difficult to get it started again even if you wish to. And you are stuck in the mud as well, which means that all your prospects look dim for getting back on the road and on your way.
So if you feel bad and don't have depression you can "move on" much more easily. If you feel bad and you trigger a depressive episode (because you suffer from depression) then untangling yourself from the bad feelings is much more difficult. In addition you may experience morbid thoughts of death and suicide which in themselves are quite unpleasant. It's the intensity, difficulty of ending it and grim feelings of helplessness and hopelessness that differentiate a depressive episode from normal occurrences of bad moods and getting the blues.