Definify.com

Webster 1913 Edition

Noun.

One who treats; one who handles, or discourses on, a subject; also, one who entertains.

Noun.

One that treats; one that handles or discourses on; one that entertains.

treater ‎(plural treaters)

Anything that treats.
A vessel used to treat oil-water emulsions by any of several mechanisms so that the oil can be accepted by the pipeline or transport.