For now you can think of hackathons as short term competitions like weekend ones, or competitions having a restricted access. Whereas the competitions page has competitions which are long term, like 2 months or more.

Hackathon is used to describe an event where programmers collaborate to solve a particular problem while a competition is an event where individual contest for a prize.

Hackathon is solution centric and collaborative while competition is prize centric and individualistc.

Although these words are always used interchangeably or as different nomenclature given to different event, they differs in approach and intent.

Thanks