Undead means dead but still walking. The undead, an undead army, undead creatures.
It is fantasy and horror vocabulary covering vampires, zombies, ghouls and skeletons — anything animated after death. The undead, as a plural noun, means those creatures collectively. Bram Stoker popularised the word in Dracula in 1897, though it existed earlier. Note that the un- here does not simply negate: undead does not mean alive, which is what makes the word useful and slightly unsettling.
1adjectiveundeadDead but still walking.
“In the film, undead soldiers rise from their graves and march on the city.”
undead is a state between life and death.
The word is un- plus dead and existed in Middle English meaning simply alive. Stoker revived it in Dracula with the modern sense, and the whole genre followed him.
Middle English
undede
English, today
undead
From Middle English undede, equivalent to un- + dead. The first attestation is from around 1400. The term was revived, popularized, and imbued with supernatural connotations by its usage in Bram Stoker's novel Dracula (1897).
Etymology from Wiktionary, CC BY-SA