An inert material was any material that was not reactive. Inert materials included inert gases.
In 2267, the USS Enterprise's transporter chief tested the ship's transporters by beaming inert material to Taurus II and back. The material returned in a disassociated condition, and the chief recommended not attempting to beam people down. (TOS: "The Galileo Seven")
In 2268, after analyzing the virus with which quadrotriticale stores on Deep Space Station K-7 had been contaminated, Doctor Leonard McCoy determined that the virus transformed into an inert material in the bloodstream, subsequently killing the tribbles by cutting off their nourishment. (TOS: "The Trouble with Tribbles")