Destructive Interference:Occurs when two or more waves superpose to form a resultant wave with a smaller amplitude than any of the individual waves. This happens when their phase difference is an odd multiple of $\pi$ radians.
Superposition Principle:States that when two or more waves overlap in space, the resulting disturbance at any point is equal to the sum of disturbances from each individual wave.
Phase Difference:$\Delta \phi$; The amount by which one wave lags behind or leads another wave. It determines whether interference will be constructive or destructive.