A tug was any vacuum-only manned or unmanned spacecraft used for pushing or pulling other spacecraft, various cargo and debris into orbit, or onto certain trajectories. Most of them were small space vehicles, up to the size of a shuttle, or even smaller. The unmanned versions were frequently robotic.
The name for these spacecraft is an obvious reference to tugboats and other vehicles similar in function.
Space tugs or orbital tugs are neither a new or untested concept in real world astronautics, though it is true that no dedicated tug-like space vehicles have been built to date. Their future implementation is often considered crucial for orbital infrastructure and industries and related space settlement efforts.