BPSK, a mechanism of modulation is expanded Binary Phase Shift Keying.

BPSK is the simplest form of Phase Shift Keying. It uses two phases, separated by 180°, and so can also be termed 2-PSK. It does not particularly matter exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at 0° and 180°. This modulation is the most robust of all the PSKs since it takes the highest level of noise or distortion to make the demodulator reach an incorrect decision.

Source: Wikipedia.