An atomic orbital is a mathematical function that describes the wave-like behavior of an electron in an atom. Specifically, atomic orbitals are the possible quantam states of an individual electron in the electron cloud around a single atom, as described by the function.
Orbitals are given names in the form:
where X is the energy level corresponding to the principal quantam number n, type is a lower-case letter denoting the shape or subshell of the orbital and it corresponds to the angular quantam number l, and y is the number of electrons in that orbital.
For example, the orbital 1s2 (pronounced "one ess two") has two electrons and is the lowest energy level (n = 1) and has an angular quantum number of l = 0. In some cases, the principal quantum number is given a letter associated with it. For n = 1, 2, 3, 4, 5 ....., the letters associated with those numbers are K, L, M, N, O .... respectively.