I'm going to stick my oar in here, just for the hell of it.
A big knife can be used for the same jobs as a machete.
There are specialized types of machete for certain jobs, and although they are all recognizeable as machetes of some kind, they look and perform a little different from each other.
My idea of a machete is defined by blade length, blade shape, and blade thickness. What I think of as a machete is a long, thin blade for chopping stuff with stalks that are less than 1/4" in diameter. It should have a wide chord, say 1.5" or more.
There is plenty of crossover with other categories, though. There are knives with machete-shaped blades, and machetes with knife-shaped blades. Marbles makes a bowie-shaped machete, just to confuse the issue.
So, for me, a machete is defined by a set of features, and it's intended use. Using it for something different does not change it's name, but altering any one of it's characteristics too much makes it something different. A thicker blade makes a Grosse Messer, Falchion, or Bolo Knife, depending on the blade shape. Making it shorter makes it a butcher's knife, or a bushcraft knife.
I could go on, but I'm starting to confuse myself, too. The best thing I could say is that you know a machete when you see it. If you're not sure, then it's probably something else. But what else exactly, is up to the individual to decide.