Q.Define amplitude and wavelength of a wave.

[!TLDR] Amplitude is a particle's maximum displacement from rest; wavelength is the distance between two successive particles vibrating in the same phase. [!ANSWER] Amplitude = max. displace …

Amplitude (A) is the largest displacement any particle of the medium undergoes, as the wave passes through it, measured from that particle's own rest (equilibrium) position; its SI unit is the metre. Wavelength (λ) is the distance between two successive particles of the medium that are in exactly the same state of vibration (the same phase) at a given instant -- equivalently, the separation between two particles whose phase differs by exactly $2\pi^c$ (section 8.2); …

Defining wavelength loosely as 'the length of one wave' without the precise same-phase-particles definition, which is what actually lets wavelength …