"Bias" by itself doesn't mean much. But it also depends on what you simulate, so we use this term in a generic way. If you apply a voltage to the two electrodes, the difference is a bias. If your system is constructed such that the two electrodes correspond to source and drain, then the bias is Vdd. If it's like a nanowire, you can add a gate, and apply a bias to this gate, which then is Vg and you can look at the transconductance, subthreshold swing, etc.
However, you can also simulate a gate stack as a twoprobe device, and then the bias is really Vg and the I-V curve would be for the leakage current.
