Depression is a "whole body" illness, involving your mood, body, thoughts and behavior. It is not just a temporary mood, nor a sign of personal weakness. We all have a not so good day once in a while.  But if your mood doesn't stay good or bounce back, you may be depressed. 

A Simple Test you can do right now:

Rate your general mood for the last week on a scale from 0 to 100.  10 or lower means you wish a dump truck would run over you and put you out of your misery.  90 is so good you think others should be jealous.  50 is in the middle.  If your mood is frequently below 45, or if your mood just occasionally feels in the 60's but drops back below 40, then you are likely depressed.  The set point on your mood may be too low, like a thermostat that is set too low.  When that happens it is very hard to stay feeling good.

Another very simple test (for adults):  How old do you feel?

If you feel younger than your age, you are probably not depressed.  If you feel your age or older, you may be depressed.  If you feel significantly older than your age, you are almost certainly depressed.