A sink is bowl-shaped fixture a basin used for holding water for washing hands or small objects such as food, dishes. In the bathroom a sink is commonly known as lavatory.
Why are sinks called sinks? Our English word basin comes from the old French word baçon, meaning shallow vessel or dish. Those basins were just basins and didn't have a drain or plug or other conveniences. What we call sinks are in fact 'basins' from which the water 'sinks'.