An AC Adapter is a cord that is used to charge or power up all of different Nintendo systems. The mainstream consoles would such as the Wii models required it to be powered up, whilst handheld consoles like the 3DS and Switch require it to charge their batteries; therefore it is an essential accessory.
For the handheld consoles, the charging variants of the adapter were actually introduced in 2003 for the Game Boy Advance SP, an upgraded model of the standard GBA console with its clamshell design and rechargeable battery. The previous Game Boy models relied on AA battery packs which were uneconomical as they had to be constantly replaced.
The design of the AC adapters had varied over the years, depending on the console they were designed for. The GBA SP's adapter was a rounded, thick black plug whilst the original iterations of the DS models used a dark grey but skinnier plug.