It's "sham stimulation" (that's the technical term used in research) and, in the case of direct current stimulation, it means what you described. The current used for tDCS is too low to be perceived, so no current is an acceptable placebo.
Ah, now I know! Serves me right for not reading the original paper before criticizing their experimental method. Thanks for the technical term. I see that the sham stimulation can still incorporate some ramp-up and ramp-down current, but I'll have to dig deeper to understand how those brief currents (or lack thereof) affect the quality of the control and the specifics of this experiment.
