Wikipedia also notes a third "culinary" usage, where "vegetable" means "any edible part of a plant with a savory flavor"; I'm not sure what that even means - presumably the usage of "savory" is not the colloquial one!
It basically means the kind of plants that you use in a main dish in traditional cooking.
You can use apples in a salad but you don't serve spaghetti with apple soup.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread is at almost 500 questions in about a month, so I think it's time for a new one.
Also, I have a new "stupid" question.