I think the two key elements that are often missing when people use the term casually are anhedonia and persistence. The wikipedia article has details about the

I think the two key elements that are often missing when people use the term casually are anhedonia and persistence. The wikipedia article has details about the accepted clinical definition: https://en.wikipedia.org/wiki/Major_depressive_disorder https://en.wikipedia.org/wiki/Major_depressive_disorder