The ratio of the tubes plate current to its grid voltage. The unit of transconductance is the "mho", which is measured in amps/volt, and is not surprisingly "ohm" spelled backwards, because one ohm is equal to one volt divided by one amp, so the unit of resistance, the ohm, is a volt/amp. Transconductance is one "figure of merit" for a tube. Higher transconductances mean higher gains and greater amplification from the tube.