Portal is a single-player first-person puzzle game developed by Valve. The game was released in a bundle package known as The Orange Box for PC and Xbox 360 on October 9, 2007, and for the PlayStation 3 on December 11, 2007. The Microsoft Windows version of the game is also available for download separately through Valve's online delivery platform Steam and was released as a standalone retail product on April 9, 2008. The game consists of a series of puzzles which must be solved by teleporting the player's character and other simple objects using the Aperture Science Handheld Portal Device. The goal of each chamber is to reach an exit point, represented by a circular elevator. The "portal gun" and the unusual physics it creates are the emphasis of this game.
In Portal, the player controls Chell (as she is named in the game credits), a test subject in the Aperture Science Enrichment Center. Gameplay revolves around the Aperture Science Handheld Portal Device (also known as the "Portal Gun", or ASHPD, the acronym), a handheld device that can create an inter-spatial portal between flat planes, allowing instant travel and a visual and physical connection between any two different locations in 3D space. Portal ends are restricted to planar surfaces, but if the portal ends are on different planes, bizarre twists in geometry and gravity can occur, such as the player walking into the portal through a wall and "falling" up out of the floor several feet behind where she started. An important aspect is that objects retain their momentum as they pass through the portals: an object that falls some distance before entering a portal will continue moving at that same speed out of the other end. This allows the player to launch objects, including Chell, up to higher levels that lack appropriate portal surfaces. Only two portal ends may be open at a time, one orange and one blue in color. If a new portal end is created, it replaces the previous portal of the same color. Either color may be used as an entrance or exit portal. The portal gun is also used to pick up objects in a similar manner to the Zero Point Energy Field Manipulator (Half-Life 2), although it cannot propel objects or pull them from afar like the Gravity Gun can.
Guided by a supercomputer named GLaDOS (an acronym for Genetic Lifeform and Disk Operating System, voiced by Ellen McLain), players use the portal gun to perform a variety of tests, such as creating portals to knock over turrets and moving to a previously unreachable area.
In their initial preview of Portal, GameSpot gave an example of a gameplay scenario: In other situations, the player may be under fire by a gun sentry. So all the player needs to do is shoot a portal open over the gun, then shoot a portal open beneath a crate, then watch the crate fall through the hole and crush the gun. It gets even crazier, and the diagrams shown in the trailer showed some incredibly crazy things that the player can attempt, like creating a series of Portals so that the player is constantly chasing herself.
Two additional modes are unlocked upon completion of the main game. In Challenge mode, the player has to get through a test in either as little time, with the least number of portals, or as few footsteps as possible. In Advanced mode, the hardest levels of the game are made even harder with the addition of more obstacles and hazards.