Couplets are any two lines working as a unit, whether they comprise a single stanza or are part of a larger stanza. Most couplets rhyme, but they do not have to. There are several set forms of the couplet and a myriad of variations based on line length and meter.
‘Closed couplets’ are couplets whose sense does not extended beyond the end of the line, as in this famous one by Alexander Pope:
True wit is nature to advantage dressed
What oft was thought, but ne'er so well express’d
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