What is circle with line through it?

A circle with a line through it is a symbol used in mathematics to represent a null set or an empty set. This symbol is often used to denote that a set has no elements in it. In set theory, the null set is denoted as the set containing no elements, and is also sometimes called the empty set. The symbol for a circle with a line through it is also used in other contexts to represent different concepts, such as prohibition or negation.