Everyone can agree that certain things are necessary for survival: food, water, shelter. Some people might add chocolate to that list. Necessary refers to anything you can't do without.
Necessary, cede, and cease all come from the same root, cedere, which is Latin for "to go away" or "to give something up." When something's necessary, you won't give it up. That fudge-covered brownie sundae from your favorite ice-cream shop? It might seem like a decadent treat, but if you're a true chocolate lover you'll insist that it's as necessary as a warm jacket in winter.