Other forms: unseens
Unseen things are either not noticed or hidden from view, like the unseen subway conductor whose muffled voice you hear making announcements about train delays.
If your goal is to be unseen as you sneak in late to your cousin's wedding, you're hoping that no one observes you. Other things are unseen simply because it's not possible to see them: "She was aware of the ghost as a chilly presence in the room, although it remained unseen." The adjective unseen is formed by adding the prefix un-, "not," to seen.