A Sake

1. cause, interest or account
2. purpose or end; reason
3. the benefit or regard of someone or something
4. (obsolete except in phrases) contention, strife; guilt, sin, accusation or charge

The word sake is generally used in constructions of the form "for X's sake" or "for the sake of X", where X is a noun (see the quotations above, for sake of, and for the sake of).

Garner's Modern American Usage notes it is common to write an apostrophe rather than apostrophe–ess in this construction when the noun ends in an /s/ or /z/ sound: for appearance' sake, for goodness' sake.