sennight

n. Archaic
A week.

[Middle English senight, contraction of seveniht, from Old English seofon nihta, seven nights : seofon, seven; see seven + nihta, pl. of niht, night; see night.]

(SEN-yt)

noun
A week.

Etymology
From Middle English, from Old English seofon nihta, from seofon (seven) + nihta, plural of niht (night)

Fortnight is a cousin of today's word. Twice as long as a sennight, it's a compressed form of "fourteen night".

Usage
"CHORUS: For now sad Charles unto the throne is come. First his drunk grandam fell down in a fit and died, And then a sennight later his Mother Queen Bit by a rabid Corgi ran naked through the streets, Froth'd at the mouth and breath'd her last. So Charles was crown'd; but now the angry mob Demand that he be brought to Tyburn Tree - Such is the curse on all odd-numbered Charleses." — Emily Sheffield; Jackdaw; The Guardian (London); May 29, 1996.

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the space of seven nights and days, week