An intro sequence is a non-interactive introductory sequence for a computer or video game. Previously, intro sequences were very often prerendered, hand drawn, or otherwise outside the main game engine. In recent years, sophisticated game engines have been able to render graphics of comparable quality to ray-traced sequences, allowing modern intro sequences to take place within the game engine (Unreal Tournament), or even as a scripted subsection of the game (Half Life).
The term "intro sequence" is used almost exclusively in the game industry. The non-abbreviated term introductory sequence usually refers to the opening of a film. The game term obtains meaning from the film term, as games use intro sequences to create a cinematic atmosphere.
Before the advent of powerful 3D video cards, the mismatch between what a game developer's workstation could render without time constraints, and what the target home computer could render in real time, was large enough that games would nearly always include some kind of scene or mood setting intro sequence.
The interactive movie genre, and games like Myst, whose graphics are almost completely pre-rendered, are a separate phenomenon.
Amiga CD32 games often included long, cinematic intro sequences; the hardware was no more powerful than an Amiga 1200, but the 650 Mb of space on each CD could be easily filled with pre-rendered videos. The launch titles Microcosm and Liberation featured a long cinematic video, with Super Stardust and Alien Breed: Tower Assault following later.
Frontier: Elite 2 was an early game to feature an intro sequence created entirely with the game engine.
Closely related to intro sequences are cutscenes, where normal gameplay is suspended while a short piece of exposition plays out, which may involve a change in the position of the camera (e.g. Dawn of War) or may not (e.g. Half-Life 2).
Developments in software and hardware have allowed more recent games to render their cutscenes within the game engine itself.