A headache is pain you feel in your head, usually caused by tired muscles, stress, dehydration, or illness. When you have a headache, the discomfort might throb at your temples, press down on your forehead, or ache at the back of your skull. Sometimes headaches arrive after staring at a screen too long, spending hours in bright sunlight without water, or tensing up during a difficult test.

Most headaches fade after you rest, drink water, or eat something. But the word also describes a person or situation that causes frustration and stress. When your teacher says the permission slip system has become a real headache, she means it's causing unnecessary trouble and annoyance. A complicated group project with members who won't cooperate might turn into a major headache for everyone involved.

You might hear someone say that dealing with a broken computer or organizing a chaotic event gives them a headache. This usage captures how exhausting and irritating certain problems can feel, as if they're causing actual physical pain even when they're not.