The Collaborative International Dictionary
Thionyl \Thi"on*yl\, n. [Thionic + -yl.] (Chem.) The hypothetical radical SO, regarded as an essential constituent of certain sulphurous compounds; as, thionyl chloride.
Wiktionary
n. (context chemistry English) The divalent radical O=S<
Wikipedia
The thionyl group is SO, a sulfur atom plus an oxygen atom.
It occurs in compounds such as thionyl fluoride, SOF.
Thionyl chloride, SOCl, is a common reagent used in organic synthesis to convert carboxylic acids to acyl chlorides.
In organic chemistry, the thionyl group is known as a sulfoxide group or sulfinyl group, and has the general structure RS(=O)R'.
Usage examples of "thionyl".
Inhaling just a whiff of thionyl chloride can destroy lung tissue, and a drop of it can eat a pea-sized hole in a hand or finger.
We can restart thionyl chloride synthesization later, when the heat subsides.
In the Vienna Hilton, Fahd asked her to buy a couple of tonnes of thionyl chloride, a `building-block' chemical used in the manufacture of many legitimate products but also an essential basic ingredient for the manufacture of mustard gas and nerve agents such as sarin.