It could be either, or both, current and voltage.
In industrial applications, a signal is considered 4 to 20 mA.
In low voltage consumer electronics, a signal is usually considered as voltage - it is easier to measure as the circuit need not be broken.
Of course, if there is a resistor (or other impedance), which there almost always is, then voltage and current will be proportional to each other anyhow, so signal means "electricity that carries information".
