The set of mathematical functions (basis functions) used to describe the molecular orbitals. Gaussian functions predominate heavily, but occasional papers use the old "Slater" orbitals or functions, which are exponentials.

The complete set of basis functions which is used to represent the molecular orbitals. In most cases, the basis set simply consists of the relevant exponents and coefficients of Gaussian functions.

Group of numerical constants used in the wavefunctions. Named by number and type of wavefunctions used to represent an atom. Specific to atom.

The set of atomic orbitals chosen to make up the molecular orbitals from overlap considerations. The choice of basis set is not unique for a given molecule.

finite set of functions used to approximately express the Molecular Orbital wavefunction(s) of system, normally atom centered, consisting of AOs differing in local angular momentum for each atom.

A basis set in chemistry is a set of functions used to create the molecular orbitals, which are expanded as a linear combination of such functions with the weights or coefficients to be determined. Usually these functions are atomic orbitals, in that they are centered on atoms, but functions centered in bonds or lone pairs have been used as have pairs of functions centered in the two lobes of a p orbital.