A Suit was a piece of apparel worn by Pan-Humans. Some were designed for protection in hostile environments (gelfield suits), while others were dedicated towards combat. (FYT suits).
In the Culture, as with most of their technology, suits tended to be sentient, with on-board AI designed to help the wearer with a myriad of functions. This includes real-time translation or even an autopilot designed to take over when a combat suit wearer is in a fight with other AI platforms, as human reaction times are far too slow to matter.