What does the word gill mean?

1. A respiratory organ found in some aquatic animals, such as fish and certain amphibians, consisting of a feathery or filamentous structure that allows for the exchange of oxygen and carbon dioxide between the water and the animal's bloodstream.

2. A unit of measurement used in the United States and some other countries to measure the volume of liquid, especially alcoholic beverages, equal to one-fourth of a pint or about 118 milliliters.

3. A small stream or brook, especially one that flows through a valley or ravine.

4. A vertical groove or slit, especially one cut into a solid object such as a tool or weapon.