ℵ₀, pronounced "aleph-null," is a symbol used in mathematics to represent the smallest infinite cardinal number. It describes the size of sets that can be put into a one-to-one correspondence with the set of natural numbers, ℕ. This means that any set with a countable number of elements, such as the integers or rational numbers, has a cardinality of ℵ₀. In set theory, Georg Cantor introduced the concept of different sizes of infinity, with ℵ₀ being the first level. Larger infinities, like the cardinality of the real numbers, are represented by ℵ₁ and beyond. Understanding ℵ₀ helps mathematicians explore the nature of infinity and the hierarchy of infinite sets.