Premonitions are feelings of anticipation or anxiety over a future event. This ability is predominantly used by Seers. Ethan began receiving them when Sarah arrived to babysit him. According to Benny's Grandma they are supposed to start around puberty.
When a Seer receives a premonition, their eyes glimmer bleach white. During premonitions, Seers are unable to observe what is in front of them. Instead, they see visions about a particular supernatural object or being that can be useful for a future event. Or seeing a past event.