When the peaks and troughs of two waves directly match each other they are said to be in phase. When "in phase" waves of the same amplitude collide, the result is a new wave with twice the amplitude of the original two. This kind of behavior is often called constructive interference.

When two or more waves of the same frequency have their positive and negative peaks occurring at the same time.

When two or more periodic, time-varying waveforms are simultaneously at the same phase angle and trending in the same direction.

Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase".

Two or more signals of the same frequency passing through their maximum and minimum values of like polarity at the same instant.

Applied to the condition that exists when two waves of the same frequency pass through their maximum and minimum values of like polarity at the same instant.