SRAM

SRAM stands for Saved RAM. Also known as "battery". This is usually small - about a kilobyte. As the name "battery" implies, SRAM must have power constantly running through it, or it will be erased, and the smaller it is, the longer it will last. A typical battery lasts around a hundred years, so there is no need to worry about them running out.

Sonic 3 uses SRAM to save games. Sonic 3 & Knuckles also uses SRAM, but the games are stored in the Sonic 3 cartridge, not the Sonic & Knuckles cartridge.