Saucer separation

The new Galaxy Class starships launced circa 2363 consisted of two sections, the stardrive section (where the warp core was located) and the saucer section.

In times of battle, it was intended that the saucer section of the ship would seperate from the stardrive section of the ship. Carrying all the non-essential personnel, this section could travel away from the battle, while the more powerful stardrive section could fight the battle.

After the battle was won, the two could be rejoined.

We first saw this ability in the epsiode Encounter at Farpoint, Part I. It was subsequnetly seen in the episodes: