Depression is loosely defined. It's more of a broad term that refers to constant sadness (mood) and could include a nutritional deficiency, constant or frequent nausea because of bad digestion, mercury leaking from dental amalgams, constant stress, toxin overload, a recent unrecognized traumatic experience, or subconscious memories.
You have to first take notice of when it started, and then you have to also think about what other significant events happened at the time.
Don't convince yourself that you have Depression if it is only that you haven't been eating well.