The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom.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 meaning is especially valid in organic chemistry. For example, the word ether refers to an oxygen-containing compound having the general chemical structure R-O-R', where R and R' are organic residues and O is an oxygen atom. Thioether refers to an analogous compound with the general structure R-S-R' where S is a sulfur atom covalently bonded to two organic residues. 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 θειον = "sulfur" (which occurs in Greek epic poetry as θειον and may come from the same root as Latin fumus (Indo-European dh-w) and may have originally meant "fumigation substance".)
See also 
- Organosulfur compounds
- IUPAC nomenclature of organic chemistry