I've had it said to me that anyone who takes money to do a specific, assigned creative task is a hack. (Usually, they're referring to media tie-ins).
I've also heard that anyone who expects to make a living from writing is a hack.
Here's my definition of a hack.
A hack is somebody who takes money for a creative task despite having no actual enthusiasm for the task concerned. Because they have no enthusiasm, they then do a substandard job.
An actor who takes a role she thinks is insulting because she needs the work and then phones it in is a hack.
A writer who agrees to do a media tie-in for a video game they think is completely stupid and then puts in a half-hearted effort is a hack.