What differentiates a melon from a squash?

Squash, melon and cucumbers are all in the family Cucurbitaceae--Gord family if you must. As said above, they are all fruits of the plant......the seed bearing part.

It's when you go down into the family that the different genus appear which help divide them into squash, melon and cucumber. For example:

Citrullus lantanus is watermelon

Cucumis melo is muskmelon

Cucumis melo indorous has many cultivars which include Casaba and Honeydew

Cucumis melo reticulatus is cantalope

Cucumis sativus is cucumber.

Squashes generally are in the genus Curcurbita. Let's not get into their specifics, they are varied...to say the least.

So melons, squashes both winter and summer and cucumber are all members of the Cucurbitaceae--or Curburbit family. They begin to differ as we go down into the genus and species. Melon is more often thought of as the sweet fruit whereas squashes and cucumbers are not sweet.

Botanically speaking, they ARE both fruits. As is a tomato.

In the kitchen, we call melons fruits, and squashes and tomatoes vegetables because of the way we use them. The first is sweet and used more as dessert, or plain like fresh fruit, and the other two are savory, used in dishes where you don't want sweet things.

It's just semantics. They are all fruits, but we call them different things based on how we use them in the kitchen.