Two waves are said to be in antiphase if the phase shift between them is 180° or π radians

Within a cycle of oscillation, the amount by which particles in that cycle are out of step with each other can be expressed in terms of Phase difference. This is expressed in degrees from 0° to 360°, or in radians from 0 to 2π. If the difference is 180° or π then two particles are said to be in antiphase. If two interacting waves meet at a point where they are in antiphase, then destructive interference will occur.