A septet is a stanza made up of seven lines. Poets use septets in many kinds of verse, with or without a fixed rhyme scheme or meter, but the defining feature is simply the seven-line unit.
Example
Night folds its silver wings across the hill,
The garden holds one last, pale breath of rain;
A window glows, then all the streets grow still,
And stars come out like lanterns lit again.
The river darkens, patient as a dream,
While distant bells dissolve into the seam
Between the waking world and sleep's quiet stream.
Related Definitions
[n] a musical composition written for seven performers
[n] seven people considered as a unit
[n] a set of seven similar things considered as a unit
[n] seven performers or singers who perform together
[n] the cardinal number that is the sum of six and one