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In [[set theory]], the '''empty set''', usually denoted <math>\emptyset</math>, is the "set" without any members.  For example, the "set of all blue-eyed lions," the "set of all mermaids," and the "set of all people born in 1700 who are still alive" are all empty sets.

In [[set theory]], the '''empty set''', usually denoted <math>\emptyset</math>, is the set without any members.  For example, the "set of all blue-eyed lions," the "set of all mermaids," and the "set of all people born in 1700 who are still alive" are all empty sets.

The empty set is unique, since any two empty sets have precisely the same members (that is, none) and is a [[subset]] of every other set.   

The empty set is unique, since any two empty sets have precisely the same members (that is, none) and is a [[subset]] of every other set.