What is a duck in cricket?

Answer 1

A duck in cricket is when a batsman goes into bat, scores no runs, and is then out again. A golden duck is when a batsman is out on their first ball, and a diamond duck is when an opening batsman is out on the first ball of the match.

Answer 2

When a batsman is out without scoring any runs i.e. scoring 0, it is called a duck. Further if a batsman gets out on the first ball he faces and implying not scoring any run, is called a golden duck.
In test matches, when a batsman is out for duck in both innings, it is called a Pair.