Isometric: Playfield is technically two-dimensional, but drawn in an isometric view so that the game looks three-dimensional. Movement is diagonally-biased instead of straight up/down/left/right. Examples: Ultima 7-8, The Immortal, Marble Madness, Diablo, etc. Part of the Perspectives and Viewpoints category.
You can browse through games in the MobyGames database that are part of this genre via the game browser.