What's a chemical?

[tylik]

So we just went shopping at Whole Foods. Today was point and giggle (in my case point because I was giggle too hard to explain coherently) at bad mycology day... but it reminded me of another example of point and giggle theatre, this time involving produce sold as "chemical free".

Um, yeah.

So - and you're free to give multiple context dependent responses - what's a chemical? If you even express some sentiment along the lines of "I don't want X, it has too many chemicals?" what do you mean? (Or, I guess, too few, I just don't know if I've heard that.)