海冰是海水变冷时结成的浮冰。由于海水不是纯净的水，海水结冰的温度是-1.8 °C（28.8 °F）。

海冰的形状和大小都多种多样，但狭义上来说，海冰与浮在海面上的冰山是不一样的。冰山是冰架或冰川断裂后漂浮到海洋上而形成的，而不是由海水结成的冰。冰山是由紧致的冰块构成的，因此是纯净的。海冰虽然是由不纯净的海水凝结而成，但由于结冰过程中盐分被排出，所以形成的也是接近纯净的冰。然而由于盐分可能会以结晶等形式成为杂质留在冰中，所以海冰的物理性质和一般的冰会有不同。

海冰对高纬度地区以至极地地区的水文、热力循环、洋流和生态系统都有重大影响。海冰过多时可能会导致海港封港，堵塞航道，挤压船舶等问题，因此也是高纬度地区海洋灾害的一种。

Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth’s surface and about 12% of the world’s oceans. Much of the world's sea ice is enclosed within the polar ice packs in the Earth's polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the Arctic ecology, including the ocean's ecosystems. Due to the action of winds, currents and temperature fluctuations, sea ice is very dynamic, leading to a wide variety of ice types and features. Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Depending on location, sea ice expanses may also incorporate icebergs.