An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters. Although the term usually refers to ice-breaking ships, it may also refer to smaller vessels (e.g., icebreaking boats that were used on the canals of Great Britain in the days of commercial carrying).

For a ship to be considered an icebreaker, it requires three traits most normal ships lack: a strengthened hull, an ice-clearing shape, and the power to push through ice-covered waters.

To pass through ice-covered water, an icebreaker uses its momentum and power to drive its bow up onto the ice, breaking the ice under the weight of the ship. Because a buildup of broken ice in front of a ship can slow it down much more than the breaking of the ice itself, the speed of the ship is increased by having a specially designed hull to direct the broken ice around or under the vessel. The external components of the ship's propulsion system (propellers, propeller shafts, etc.) are at even greater risk of damage than the vessel's hull, so the ability of an icebreaker to propel itself onto the ice, break it, and clear the debris from its path successfully is essential for its safety.