An overheated medium is a medium that has reached total expression and undergone reversal. The "heat" is a metaphor.
Oh. That McLuhan stuff.
Yes, boy, "that McLuhan stuff." A medium is any tool or system that extends the human identity. All media exert influences on culture and mind.
Hmm. Dad, is television a medium?
No, son. McLuhan was wrong about television. Television is a force of nature like locusts and ozone. We know this now.