Nothing is the lack of anything. "Nothing" and "zero" are closely related, but not identical, concepts. The term "nothing" is rarely used mathematically, though it could be said that a set contains nothing if and only if it is the empty set, in which case its cardinality (or size) is zero.
The concept of "nothing" has been studied throughout history by philosophers and theologists; many have found that careful consideration of the notion can easily lead to the logical fallacy of reification. The understanding of "nothing" varies widely between cultures, especially between Western and Eastern cultures and philosophical traditions, though existentialism, and in particular Heidegger have brought the understandings closer together.
Informally, a person, event or object might be said to be nothing if it is particularly unimpressive.
Most of the Universe is said to be made out of nothing, but this is equally thought to be dark matter, i.e. something.
For the mathematics of nothing see the zero article.