What it is called?

One that speaks two languages - is bilingual - can be said to be diglot.

Ducks are never male. The males of the species are called drakes.

The white part of your fingernail is called the lunula.

The thin line of cloud that forms behind an aircraft at high altitudes is called a contrail.

The tendency of the leaves or petals of certain plants to assume a different position at night is called nyctitropism.

The back of the human hand is the opisthenar.

Someone who uses as few words as possible when speaking is called pauciloquent.

People that study fish are called ichthyologists.

The pin that holds a hinge together is called a pintle.