Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted Σ and Δ (the preceding superscript "1" indicates a singlet state).  The singlet states of oxygen are 158 and 95 kilojoules per mole higher in energy than the triplet ground state of oxygen. Under most common laboratory conditions, the higher energy Σ singlet state rapidly converts to the more stable, lower energy Δ singlet state. This more stable of the two excited states has its two valence electrons spin-paired in one π* orbital while the second π* orbital is empty. This state is referred to by the title term, singlet oxygen, commonly abbreviated O, to distinguish it from the triplet ground state molecule, O.
Molecular orbital theory predicts the electronic ground state denoted by the molecular term symbol Σ, and two low-lying excited singlet states with term symbols Δ and Σ.  These three electronic states differ only in the spin and the occupancy of oxygens two antibonding π-orbitals, which are degenerate (equal in energy). These two orbitals are classified as antibonding and are of higher energy. Following Hunds first rule, in the ground state, these electrons are unpaired and have like (same) spin. This open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet (Σ) ground states.
Two less stable, higher energy excited states are readily accessible from this ground state, again in accordance with Hund's first rule; the first moves one of the high energy unpaired ground state electrons from one degenerate orbital to the other, where it "flips" and pairs the other, and creates a new state, a singlet state referred to as the Δ state (a term symbol, where the preceding superscripted "1" indicates it as a singlet state). Alternatively, both electrons can remain in their degenerate ground state orbitals, but the spin of one can "flip" so that it is now opposite to the second (i.e., it is still in a separate degenerate orbital, but no longer of like spin); this also creates a new state, a singlet state referred to as the Σ state. The ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below.
The Δ singlet state is 7882.4 cm above the triplet Σ ground state., which in other units corresponds to 94.29 kJ/mol or 0.9773 eV. The Σ singlet is 13 120.9 cm (157.0 kJ/mol or 1.6268 eV) above the ground state.
Radiative  transitions  between  the  three  low-lying electronic states of oxygen are formally forbidden  as electric dipole  processes. The two singlet-triplet transitions are forbidden both because of the spin selection rule ΔS = 0 and because of the parity rule that g-g transitions are forbidden. The singlet-singlet transition between the two excited states is spin-allowed but parity-forbidden.
The lower, O(Δ) state is commonly referred to as singlet oxygen. The energy difference of 94.3 kJ/mol between ground state and singlet oxygen corresponds to a forbidden singlet-triplet transition in the near-infrared at ~1270 nm. As a consequence, singlet oxygen in the gas phase is relatively long lived (54-86 milliseconds), although interaction with solvents reduces the lifetime to microseconds or even nanoseconds. In 2021, the lifetime of airborne singlet oxygen at  air/solid interfaces was measured to be 550 microseconds.
The higher Σ state is very short lived. In the gas phase, it relaxes primarily to the ground state triplet with a mean lifetime of 11.8 s. However in solvents such as CS and CCl, it relaxes to the lower singlet Δ in milliseconds due to nonradiative decay channels.