Define crest and trough.

Hint :In a wave, the maximum value of upward displacement is called its crest. On the other hand trough is just opposite to the crest, in a wave the minimum value of downward displacement is called its trough. We can also say that the highest upward amplitude is called crest while the lowest downward amplitude called trough.

Complete Step By Step Answer:
Where the displacement of the medium is at a maximum, that point on the surface of the wave is known as crest. But the trough is just opposite to the crest, in simple words the minimum or lowest point in a cycle is known as trough.

When the crests and troughs of two sine waves of equal amplitude and frequency intersect or collide, while being in phase with each other, the result is called constructive interference and the magnitudes double (above and below the line). When in antiphase – $ {180^o}C $ out of phase – the result is destructive interference: the resulting wave is the undisturbed line having zero amplitude.

Additional Information:
There are two types of waves used in classical physics : mechanical waves and electromagnetic waves. In mechanical waves, stress and strain are the two fields which oscillate about a mechanical equilibrium. In electromagnetic waves, energy gets interchanged between electric and magnetic fields.

Note :
Wave is propagating dynamic disturbance which occurs when change in equilibrium takes place of one or more quantities. Wave is always described in wave function. In physical waves, there should be at least two field quantities involved in wave medium. Waves are periodic when these quantities oscillate frequently about a resting value at some frequency. Travelling waves are said to be those waves when the entire waveform moves in one direction.