Thio-

The prefix thio-, when applied to a chemical, such as an ion, denotes that there is at least one extra sulfur atom added to the chemical named in the root word. For example, thiocyanide is cyanide with an additional sulfur atom (note, though, that bonding patterns are different; cyanide contains a triple bond while thiocyanide has two double bonds).

The prefix 'thio-' can also be placed before the name of a specific or general compound (or even ion) to mean that an oxygen atom in the compound(s) has been replaced by a sulfur atom.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 radicals 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 radicals.

Thio- can be prefixed with di- and tri- in chemical nomenclature.

It came from Greek θειον = "sulfur" (which occurs in Greek epic poetry as θεειον and may come from the same root as Latin fumus (Indo-European dh-w) and may have originally meant "fumigation substance".)