Just a quick post on the definition of depression (okay, I don’t make quick posts!)–
I know in this world of language and assorted semantics, one word can often be used to mean many different things; the same is true for depression! I’m guilty of it too…I just said to myself: I was so depressed during that time; I actually meant: I was so sad or blue during that time.
To me, the key word here is time. I’ve looked at a lot of research and literature recently. (especially since I’m speaking and writing about depression more publicly since I came out…I don’t want to mislead or give out bad info). Every resource says that it is totally normal to feel down or blue every now and then…and for a short period of time. A few days. But, depression is different and it feels different. You feel “out of it” for a long time…probably long enough where you may not remember when it started. You are not yourself, you don’t care about the things you know you care about. You may still go through the motions, but it is different.
The NIMH (National Institute for Mental Health; NIMH.org) breaks down the types of depression this way:

Seasonal (think cold, winter, gray days),
Major depression disorder: you are not functioning; this type may require time off from work and can include a breakdown
Dysthymic disorder—goes on for 2 or more years; it is not as severe but it can prevent you from doing your normal tasks, it definitely prevents you from doing them with joy and easily (think: oh, I have to get up today; feeling like you are in a fog)
Psychotic depression—this type is coupled with psychotic symptoms, such as hallucinations or delusions
Postpartum depression—occurs up to 1 year after giving birth; can be coupled with many signs of motherhood—feeling guilty, unable to concentrate, anxiety, sadness

When you read the symptoms of depression, it is easier to understand why we often confuse them with just feeling blue or normal mood swings of life. I personally think assessing your mood and the amount of time you’ve felt that way can help you determine if you need to take a step toward intervention. Most people recommend a physical so a doctor can determine whether a physical illness could be contributing to your depression. Some start with a trusted friend; others a therapist, who can help you think through what may have triggered your depression and begin to help you put tools in place to treat it. I recommend all of these steps, as each one (and more) helped me in some way.