In telecommunication, a bipolar signal is a signal that may assume either of two polarities, neither of which is zero.
A bipolar signal may have a two-state non-return-to-zero (NRZ) or a three-state return-to-zero (RZ) binary coding scheme.
A bipolar signal is usually symmetrical with respect to zero amplituude, i.e., the absolute values of the positive and negative signal states are nominally equal.
References
Bipolar signal Wikipedia(Text) CC BY-SA