Cading: Reinforcing a building's doors or blockading the stairs. Can be both useful or griefing.
Feral: A lone player, one that does not coordinate within an organization.
Griefing: Intentionally causing trouble for one side and going against the spirit of fair play. For example, unhooking generators simply to leave them lying around, would be griefing the human side because the presence of a generator can help prevent a building from being ruined. A less clear example would be clawing out the fusebox in a lit building with other zombies inside, because this could draw humans to the building. On the other hand, zombies should be actively destroying buildings and fuseboxes. The definition of griefing can often be flexible.
Medkit: First-aid kits. Used to restore 5 HP.
Stat-padder: A person who pursues leaderboard rankings. Usually said with a negative implication.
Streetmeat: A human idling on the streets. Easy food and SP for other zombies.
Zed: Zombies