LAC is an acronym for Light Attack Craft. In some works of science fiction, they are small ships only capable of sub-light speeds, used to patrol the space around planets. LACs are typically used for piracy suppression, search & rescue, patrol and customs duties. Individually, they aren't very impressive. When a hundred of them show up, you might be in deep trouble. Some larger, heavily populated planets have thousands of them organized in wings of 100 and call the overall formation the High Guard.

Pictured below: Dylan Aerospace LX-1 single seat LAC attached to the Red Raiders wing of Parliament's High Guard.