The Second and a Half Dimension

What do Dick Tracy, 1942, Super Mario World, Space Harrier and DOOM have in common?  Not much, but one feature they do all share is a 2.5D design atheistic.  In recent years 2.5D has come to mean a video game that uses three-dimensional polygonal models restricted to two-dimensional environments.  In the past though, it was a designation given to games that would use visual tricks to create the impression of a 3D space.  Let me elaborate on this a bit further by going into some more detailed examples.

In Dick Tracy the players can move left and right, as well as up and down (via climbing, jumping, falling, etc.).  Enemies come from the sides of the screen, some in the foreground and others in the background.  Foreground enemies can be fought using a pistol or fisticuffs, while background enemies can be shot at with a Tommy gun.  World War 2 themed bullet-hell shooter, 1942 allows the player to roll and loop to avoid enemy fire by momentarily climbing to a higher altitude.  Like many top-down shooters, fire from the player's strike craft hits other airborne targets, in addition to ground emplacements and ship turrets (no dive attacks needed).  Super Mario World has a few stages with special fences that feature small rotating patches that allow Mario get on the backside.  Space Harrier, much like Afterburner and Thunder Blade, uses enlarging pixels to create the illusion of of forward momentum.  It's a technique that is also used in a lot of racing games such as Hang On and Out Run.  However, in those games the player can only move left and right via a steering input (rather than the more common two axis of motion).  Perhaps that means they are 1.5D games?

DOOM might sound like a stretch since it has all the trappings of a "3D game" (along the lines of "Wolfenstein 3D" or "Duke Nukem 3D").  Much like other aforementioned titles, it uses cleaver design to mask a simplified system of rendering.  Technically, the game has higher and lower elevations, but there can't be any vertical overlap.  One of the big reasons for this is to take pressure off the CPU by reducing the complexity of the mathematical calculations needed.  In modern games, 2D and 2.5D graphics are done purely for style or gameplay reasons, but back in the days of 16-bit gaming platforms it was a necessity due to hardware limitations.  

Maybe that's why the meaning of the term has gradually changed over time...