Correct spelling


Correct spelling, explanation: triathlon is a word constructed by compounding the prefix tri- (meaning three), with a base word athlon, which is an Old Greek word for contest. This is pretty straightforward, especially since you may know words similar to triathlon, such as biathlon or decathlon.

Definition of triathlon:
1. a sports contest consisting of three disciplines
There are different types of triathlons, but all consist of swimming, running, and cycling.
Triathlon became an Olympic sport in 2000.

Incorrect spelling


Incorrect spelling, explanation: if you’re unsure about the spelling of triathlon, but you know it’s about three athletic contests, it’s best to keep the word athlete in mind. Athlete comes from athlon, so it’s similar in structure, but athalon is not as similar to athlete, hence triathalon is incorrect.