Define constructive interference. $OR$ Define destructive interference.

(N/A) Constructive interference: When two waves of the same frequency and phase meet at a point,the resultant displacement is the sum of the individual displacements,leading to a maximum amplitude. This occurs when the path difference between the waves is an integral multiple of the wavelength,i.e.,$\Delta x = n\lambda$,where $n = 0, 1, 2, ...$.
Destructive interference: When two waves of the same frequency meet at a point such that they are out of phase by $180^{\circ}$ (or $\pi$ radians),the resultant displacement is the difference of the individual displacements,leading to a minimum amplitude. This occurs when the path difference between the waves is an odd integral multiple of half the wavelength,i.e.,$\Delta x = (2n + 1)\frac{\lambda}{2}$,where $n = 0, 1, 2, ...$.