What does UD stand for?

Update

Either short for update information or, as a verb, applying the update mechanism. See also official updates.

Unreliable Datagram

Unsolicited Downstream

Up & Down

Upper-Division

Upper Diamond

Upside Down

Urban Decay

Human migration is movement by humans from one place to another, sometimes over long distances or in large groups. Historically this movement was nomadic, often causing significant conflict with the indigenous population and their displacement or cultural assimilation.

Uglyadog