Conversely, if there are two signals of the same frequency (same distance between zero crossings on the x axis) but one signal begins earlier and the other one begins at a later point, they are said to be out of phase by some amount, i.e. there is some time difference between the two signals.
Phase difference is expressed in degrees from 0 to 360, or in radians. If the difference is 180 degrees then the two signals are said to be in antiphase: they are equal but opposite, and if added together will sum to zero. If the phase difference is 90 degrees then the signals are said to be in quadrature.