A party for women only, usually one held for a woman before she is married.
The male equivalent is called a stag party (or stag night)
Source: Cambridge Advanced Learner's Dictionary
Etymology: hen: The original word survives in German (Hahn "cock"), Swed., Dan., etc.; extension to "female of any bird species" is c.1325 in Eng. Hen as slang for "woman" dates from 1626; hence hen party "gathering of women," first recorded 1887. Henpecked is from 1680.
"The henpect Man rides behind his Wife, and lets her wear the Spurs and govern the Reins." (Samuel Butler)
henpecked husband = Pantoffelheld
IMPRESS YOUR FRIENDS TODAY say something like:
"Stag and hen parties and probably satisfy a deep instinctive need for male and female group bonding - don't you think?"