What is Photosensitisation, singlet oxygen?

A photosensitiser is a molecule that absorbs light and transfers the energy or an electron to another molecule, which does not itself absorb: this is photosensitisation. In triplet energy transfer the triplet state of the sensitiser, formed by intersystem crossing, gives its energy to the acceptor on contact, leaving the acceptor in its triplet state. Singlet oxygen is the lowest excited state of $\ce{O2}$, ${}^1\Delta_g$, formed this way from ground-state ${}^3\Sigma_g^-$ oxygen.