When I talked about face space at Upsideclown back in July 2000, I had no idea that I was talking about something real and that "face space" was a well established term. And when I talked about the basis vectors of face space -- I mean, yes, I knew that people were doing research here, but I didn't really expect to find pictures of faces constructed from eigenfaces [from Shape Analysis in Computer Vision].
Steve DiPaola in Investigating Face Space explains the concept very well: "Imagine an n-dimensional space describing every conceivable humanoid face, where each dimension represents a different facial characteristic". Then you can reduce this space along dimensions of similarity to, say, only 25 basis vectors -- that is, the eigenfaces, which can be added together to produce new, different, faces. And what I really didn't expect is that said concept would be used in a game, to wit The Sims, as a face generator.