Singlet oxygen is an electronically excited state of molecular oxygen that is less stable compared to its ground state. In singlet oxygen, one of the unpaired electrons undergoes a change in its spin, resulting in two electrons with opposite spins. These paired electrons can occupy separate antibonding orbitals or the same orbital. 

Triplet oxygen refers to the electronic ground state of molecular oxygen (O2), characterized by a configuration with two unpaired electrons, resulting in a total spin quantum number (S) of 1. It is more stable than a singlet oxygen.