1. Literally, to go beneath the surface of water or some other liquid. The storm punched a hole in the hull of the vessel, causing it to sink below the waves. You might expect such a heavy object to sink below the surface of any liquid, but because it is less dense than mercury, it actually floats on top of it.

2. To move or appear to move beneath or behind some object or boundary. We sat watching the sun sink below the horizon. The child sank below the table to avoid having to talk to the people his parents had invited for dinner.

3. To reach or go to a level or degree beneath some point or threshold. Temperatures are expected to sink below freezing tonight, so beware of icy roads in the morning. We can't let the power output sink below 500 kilowatts.