Major Depressive Disorder
A discrete episode of persistently low mood and/or loss of interest lasting at least two weeks, accompanied by a cluster of physical, cognitive, and behavioral changes severe enough to impair functioning.
Major Depressive Disorder vs. Persistent Depressive Disorder (Dysthymia)
Both involve depressed mood and related symptoms, but MDD centers on a discrete episode while PDD centers on a chronic, lower-grade baseline that has become the person's "normal."
Onset / developmental pattern
MDD can begin at any point and is marked by a relatively identifiable start to the episode; PDD often has an insidious, early onset that the person may struggle to date precisely.
Duration / course
MDD requires at least two weeks per episode; PDD requires at least two years in adults (one year in youth) with no more than two consecutive symptom-free months.
Associated features
MDD episodes are often more acutely intense (marked anhedonia, significant vegetative symptoms); PDD symptoms are typically less acutely severe at any given moment but persist far longer.
Functional impairment
MDD impairment tends to cluster around the episode itself; PDD impairment accumulates gradually across years and may be underestimated because the person has adapted their life around the chronic low mood.
Rule-outs / exclusions
Both require ruling out a manic/hypomanic history (which would point toward a bipolar-spectrum condition instead) and substance/medical causes.
The differential question
Ask: is this a change from a clearer, better-functioning baseline that started at an identifiable point (favors MDD), or has the person felt this way, to varying degrees, for years without a clear symptom-free stretch (favors PDD)? The two are not mutually exclusive -- a major episode can be superimposed on a persistent dysthymic baseline ("double depression").
Common student confusion
Students often assume PDD is simply "a milder form of MDD." Severity and duration are separate dimensions: PDD is defined by chronicity, not by being categorically less serious.
Case clues
Watch for duration language: "since early adolescence" or "for years, as long as I can remember" points toward PDD; "for the past three weeks, this is really unlike me" points toward MDD.
Read each disorder in full
Persistent Depressive Disorder (Dysthymia)
A chronic, lower-grade depressive mood present for most of the day, more days than not, for at least two years in adults (one year in children/adolescents) -- persistence rather than intensity is the defining feature.