1. a substantial part of a larger project or goal
the worst part is done, that's half the battle
The idiomatic phrase 'half the battle' can be used literally or sarcastically to refer to the main or most challenging part of a larger goal or project. If you say that something is half the battle it means that it is worst or most difficult part. This phrase comes from the full idioms which not often used anymore: 'the first blow is half the battle' or 'a good start is half the battle'.