An entity that cats can see but humans can't. Cats are driven insane by them and must destroy them. Whenever you see a cat attack nothing with an insane fury, it's probably a greeble
In the Lego online community, greeblies are the intricate details on the sides of spaceships and sci-fi MOCs that define the mechanics and workings of what the MOC is meant to be. They were famously used in the giant Star Destroyer Lego set a few years ago, and also in the more recent Death Star II set. Plates with rails are often used for greeblies because they have a protrusion that only sticks out half a stud without needing a jumper plate. (source: Brickspace.org)
Random bumps, extrusions, and hills and valleys, usually planar. Used in 3D modeling and animation to add interesting detail, like that on the surface of the DeathStar.
Greeblin - Often considered skrunkly; a creature or person who resembles a traditional portrayal of a gremlin, but who is fixated on doing a seemingly pointless tasks or fulfilling an eccentric desire via often greedy acquisition.
Often paired with the verb to greeble, meaning to act silly and fulfill eccentric desires.
Dave: Did you see my cat greeblinglast night? He sucked on Cindy's finger for over 20 minutes with the weirdest look on his face.
Walter: Wow, what an absolute greeblin.