The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom. This term is often used in organic chemistry. For example, from the word ether, referring to an oxygen-containing compound having the general chemical structure R–O–R′, where R and R′ are organic functional groups and O is an oxygen atom, comes the word thioether, which refers to an analogous compound with the general structure R–S–R′, where S is a sulfur atom covalently bonded to two organic groups. A chemical reaction involving the replacement of oxygen to sulfur is called thionation or thiation.
Thio- can be prefixed with di- and tri- in chemical nomenclature.
The word derives from Greek θεῖον theîon = "sulfur" (which occurs in Greek epic poetry as θέ(ϝ)ειον théweion and may come from the same root as Latin fumus (Indo-European dh-w) and may have originally meant "fumigation substance".)