Saucer separation is the procedure by which a Galaxy-class starship splits into its saucer and stardrive sections, usually to protect civilians while the warship half goes into danger.
The Galaxy-class was designed to carry families, so the saucer could detach and move away while the stardrive section, which held the warp engines and main weapons, faced a threat. Captain Picard first used the maneuver during the Enterprise-D's initial mission to Farpoint Station. Geordi La Forge later commanded the stardrive section in a battle against an automated weapon system on Minos.
The procedure took on dramatic importance when a warp core breach doomed the Enterprise-D. The crew separated the saucer just in time, but the shockwave from the exploding stardrive sent it crashing onto the surface of Veridian III, where the ship was lost.
Captain Kirk once ordered preparations to separate the original Enterprise's saucer in an emergency, though the maneuver was not carried out. On the Enterprise-D, the separated stardrive section was commanded from a dedicated battle bridge.