Symmetry of MO's
 
Pi (π) molecular orbitals being antisymmetric with respect to a defining plane containing at least one atom. In the case of two-centre bonds, a π bond has a nodal plane that includes the internuclear bond axis.
 
Sigma (σ) molecular orbitals are symmetric with respect to a defining plane containing at least one atom. In the case of two-centre bonds, a σ bond has no such nodal plane.
 
A MO that has cyllindrical symmetry around the internuclear axis is a σ orbital. For example wavefunction of the H2+ ion can be constructed as ψ = ψ_1s(A) + ψ_qs(B). Such an orbital is denoted as a 1sσ orbital as the σ orbital originates from 1s orbitals. The anti-bonding combination ψ = ψ_1s(A) - ψ_qs(B) is denotes as 1sσ^*.
 
Sources:
http://www.chemicool.com/definition/sigma_pi.html
http://goldbook.iupac.org/O04320.html
Atkins Physical chemistry