I find it quite annoying that alot of people at my school and town misuse the word "depression" , for example "I feel so depressed today." or "I suddenly had a moment of depression.".
If you just had a bad day or your lover just broke up with you, it does not mean you are depressed.
"Clinical depression (also called major-depressive disorder or unipolar depression) is a common psychiatric disorder, characterized by a persistent lowering of mood, loss of interest in usual activities and diminished ability to experience pleasure."
-Wikipedia
This means that a depression is long-term and if you read this Wikipedia article you will understand that it's caused by lack of neurotransmitters. This can be heritable, but also trauma's can ofcourse have effect on it.
I know that definitions become definitions because of many people using a word, setting a trend and the dictionary takes it over. But people who really are depressed can feel pretty offended by someone with a great life whose hamster has just died saying "Gosh, I actually feel pretty depressed about its dead".
Other words you can use: Gloomy, sad, sombre, pensive, triste, moody, cheerless, joyless, wistful, dreary, dull, despondent and low spirited.
By the way, I personally never have been depressed.