Bipolar signal

In telecommunication, a bipolar signal is a signal that may assume either of two polarities, neither of which is zero.

Note 1: A bipolar signal may have a two-state non-return-to-zero (NRZ) or a three-state return-to-zero (RZ) binary coding scheme.

Note 2: A bipolar signal is usually symmetrical with respect to zero amplitude, i.e., the absolute values of the positive and negative signal states are nominally equal.

Source: from Federal Standard 1037C and from MIL-STD-188