(noun) - A binder is the agent in paint that serves -- on the molecular level -- to make the paint stick to the support (that is, the canvas, board, or what have you).
For example, the binder used in watercolor and gouache is gum arabic. Tempera -- real tempera -- uses egg. Acrylic paint uses acrylic as its binder. And oil paints use (you guessed it!) some sort of drying oil, usually linseed or a modified oil called an alkyd. It's great, the way chemistry works in paint ... even if one (raising hand) doesn't necessarily understand chemistry.