An S-class shuttle was a type of small spacecraft used in the Federation, considered antiquated by the late 24th century. It had two seats, no warp drive, and manual helm controls. Tom Paris' first flight was on an old S-class shuttle, with his father at the age of eight.
In 2376, Paris recalled to the sentient starship Alice that he had been scared out of his wits at first, when he couldn't get the shuttle level, but then there was a "moment of clarity, when suddenly everything made sense". (VOY: "Alice")