Undead were, in a broad sense, any form of deceased beings or bodies which, through re-animation, continued to exist and manifest in Nosgoth's Material Realm. They were distinct from wraiths, which were denizens of the Spectral Realm.
In Blood Omen: Legacy of Kain, they were more specifically definable by their characteristic blood. The Vampires were sometimes referred to as undead, but they did not share in this trait. Later games introduced other creatures and entities also suitable to this classification.