What is the definition or description of: situational depression?

It just means: The depression is caused by a specific situation that is plausibly related to feeling depressed. Situational depression is usually easier to resolve than depression that seems unrelated to any life circumstances.

Depression: Situational depression is as the name suggests, a mood disorder with sadness, negative thoughts , and dysphoric mood along with many other classical symptoms of major depression, with one exception. The cause is related clearly to some external event, or even internal, say an acute distressing medical condition. It is the situation, whether internal or external, that is causing the depression.