Linked from
The 141 pages that link to Wavelength, each with the reason it gives.
DiffractionNarrower topic: It sets the scale against which an opening or obstacle must be compared.
InfraredNarrower topic: Infrared is defined by its position at longer wavelengths than visible red.
X unitNarrower topic: The X unit measures this property of X-rays.
De Broglie equationNarrower topic: The equation assigns this general wave property to a particle.