IPA: /hɛpˈtæθlən/
KK: /ˈhɛpˌtæθlən/
A sports competition that includes seven different events, usually for women, such as running, jumping, and throwing.
She trained hard for the heptathlon to improve her performance in all seven events.
Heptathlon is formed from "hepta-" (from Greek "heptá", meaning seven) and "athlon" (from Greek "áthlon", meaning contest or prize). The word refers to a contest consisting of seven different track and field events.
Think of 'seven' ('hepta-') events in a 'contest' ('athlon') — that's what a heptathlon is.