No, it's not the start of a joke!

I'm just wondering why are exact ODEs called exact?

All I can think of so far is that they don't end up with an integration constant on one of the variables.

But as the solution is e.g. f(x,y) = c it's still going to represent a family of solutions.

Any ideas? It's probably really obvious (Worried) It's been annoying me for a few days now.

Thanks and merry Christmas and new year to you all!