An electron orbital (often referred to as just orbital) is a wave-like description of a potential position of an electron comprising part of an atom (atomic orbital) or molecule (molecular orbital). The term orbital derives from the idea of the electron orbiting the nucleus, but quantum mechanics doesn't model it as a planet-like orbit, but as a standing wave. Quantum mechanics limits the possible ways an electron can be bound to the nucleus, and the term orbital refers to which specific manner the electron can be bound. The orbital(s) occupied by the electron(s) of an atom comprise its state of excitation.
Each orbital is described by three quantum numbers, representing energy (principle quantum number), angular momentum (azimuthal quantum number), and the angular momentum vector (magnetic quantum number), termed n, l, and m respectively. (Spin is a fourth quantum number affecting electrons, designated ms, a type of angular momentum associated with a single particle). A common designation for the first two quantum numbers of an orbital lists the numerical value of n followed by a letter code for l: s, p, d, and f for l=0, 1, 2, and 3 respectively. Thus the symbol 1s indicates n=1 and l=0. A superscript can be used to indicate the number of electrons in an atom with those numbers, e.g., 1s². The set of orbitals with a specific principle quantum number are known as a shell, i.e., electron shell.
Molecular orbitals have additional complication, there being more ways an electron can be bound.
Specification of orbitals are used in spectrography to describe the specifics of the excitation or relaxation that produces a spectral line, e.g., a particular frequency of an emission line might be due to the relaxation of an electron from one specific orbital to another. Each orbital (for a specific nucleus) has an associated energy level (e.g., the binding energy of the electron in that orbital), and a photon emitted has the photon energy (thus the electromagnetic radiation frequency) equal to the energy-difference between the former and latter orbital. The energy level depends by far the most on n and a change in n is associated with a typical spectral line. For example Lyman alpha emission can be produced by an electron's fall from the state 2p to 1s. However there are slight differences in the electron's energy level due to other factors, such as differences in the other quantum numbers. For example, a change in spin is the source of the 21-cm line.