A headache is a pain or ache in your head, especially one that lasts longer than a few minutes. You might get a headache after a day of staring at a computer screen.
There are many reasons for getting a headache, from staying up too late to a stressful day to skipping your morning cup of coffee. A more serious type of headache, like a migraine, might keep you home from work for a day, but most headaches are just a nuisance. You can also describe a problem or struggle as a figurative headache: "Learning Danish is turning out to be a real headache." In Old English, the word was heafodece.