Wavelength

Wavelength ($\lambda$) is the minimum distance between two points in phase (e.g. crest to crest, trough to trough, compression to compression).

Related to frequency and speed by:

$\lambda = \frac{v}{f}$

Long wavelength = low frequency (and vice versa) for a given wave speed.

Wavelength determines the type of electromagnetic radiation (e.g. radio waves have wavelengths from metres to kilometres; gamma rays have wavelengths less than $10^{-12}$ m).