Deflector dish

The Enterprise-D deflector dish, modified into an anti-Borg weapon

The deflector dish is a feature usually seen on Federation starships, facing foreward. The dish projects a force field ahead of the ship that deflects small debris out of the ship's path while it is travelling at warp speed. This field protects the ship from warp-speed collisions with micro-meteoroids and other small objects, since even interstellar dust could be harmful to the ship at such high speeds.

The defector dish can also function as a weapon. In TNG "Best of Both Worlds", the crew of the Enterprise modified the ship's dish to project a blast of high-frequency energy, and similar feats have been seen on other occasions since then.

The starships of other major Star Trek civilizations usually do not have highly-visible deflector dishes, but they presumably have a less obvious mechanism that performs this function.