With reference to
games consoles, a device that allows cartridges or discs from one
teritory to play on a console from another. Eg, playing a Japanese game on a UK
SNES. For cartridge based consoles, the adaptor fits between the console and the cartridge, in some cases (eg, SNES) requiring a second cartridge to present the console with local key. Sold in import shops. In disc based systems, the adapto plugs into an expansion slot on the console.
A quick explaination of why these things exist: The console manufacturers in Japan, instead of distributing their consoles and games themselves in other countries, form subsidury companies or licence the distribution rights to third parties. Since a second company is then involved, competition between the parent company and the subsidury or licencee is undesirable. However, because games generally come out in Japan before elsewhere in the world, people import the games, taking money away from the local company. This is why consoles are built with teritorial lockouts.
Sometimes the lockout takes the form of a software check that tests the state of jumpers or reads the teritory from a ROM. This can be also be defeated using a teritory switch. Other times, the lockout is a hardware device in the console that will refuse to allow the console to boot if it isn't presented with the correct key in the cartridge or code on the disc. For the PC Engine / Turbo Grafx 16, the pins on the Japanese and US cards are in a different order.