A migraine is a severe headache. When you get a migraine, you'll probably want to stay in bed, in a dark room, until it passes.
When you get a headache that's so painful that you can't work or function, you've probably got a migraine. It's usually women who get migraines, and you are either a person who gets migraines throughout your life, or you never get one at all. Migraines often come with side effects like nausea and blurred vision, and they often affect just one side of the head. The word comes from the Greek hemi-, "half" and kranion, "skull."