An aura is a sensation which precedes an attack, such as an epileptic seizure or a migraine headache. In epilepsy, the aura may precede the seizure by a few seconds or several hours. The aura may include visual, auditory or olfactory hallucinations (e.g. a particular smell), or just a "feeling" that a seizure is coming. In migraine, the aura immediately precedes the attack, and is usually visual (e.g. a halo of light around the visual field).

The mechanisms causing an aura are poorly understood. However, many people who suffer severe migraines or severe epilepsy can use their aura as a warning sign which may give them time to prepare for or even ward off a full attack.