A portal is a gateway that connects location 'A' to location 'B' through magical or technological means.
Common examples include teleportation portals, demon-summoning portals, and portals to other dimensions.
The games Portal and Portal 2 (both by Valve) very often features portals, however, it is very important to note that this tag is not used to describe the game, but is used to describe the main feature of the game. The game itself should be tagged portal_(series). Many pictures will contain both of these tags.