Nintendo Wi-Fi Connection spoofing, including GTS spoofing is an exploit for Generation IV and Generation V Nintendo DS Pokémon games and for online services in other Nintendo DS games in general.
This exploit allows the player to connect to a custom Wi-Fi server, where they can obtain Pokémon that were generated illegitimately, trade with other players on a custom server, host Battle Videos and battle other players online.
In the past Nintendo Wi-Fi Connection spoofing could easily be achieved by changing the "Auto-obtain DNS" setting to "No" and entering a custom Primary DNS (to a server that offers an unofficial Wi-Fi service) on the Nintendo Wi-Fi Connection Setup menu.
After Nintendo's discontinuation of Nintendo Wi-Fi Connection on May 20, 2014 the only way to use the exploit is in combination with a series of game specific cheat codes (e.g. by using an Action Replay), which makes the exploit a little less useful.