Wavelength
Also called Lambda
Wavelength is the distance between two successive corresponding positions on a wave, such as peak to peak or trough to trough. It is a length, measured in meters, and written with the Greek letter lambda.
Symbol , unit the meter. AP Physics 2 EK 14.2.A.1.vi puts it as the distance between successive corresponding positions, such as peaks or troughs, on a wave. Any pair of matching points does: peak to peak, trough to trough, or one upward zero crossing to the next.
Longitudinal waves have no peaks, so measure compression to compression or rarefaction to rarefaction instead. The idea is identical, the picture is not.
Wavelength belongs to the wave in a medium, not to the source. Cross a boundary and holds fixed (EK 14.3.A.1.iv) while changes, so has to change with it. Light entering glass slows and its wavelength shortens; its color does not change, because color follows frequency.
Read the axis before you read the graph. A displacement-against-position graph shows as the horizontal repeat distance. A displacement-against-time graph of the same wave shows the period. Same shape, different quantity, and swapping them is the standard error on wave graph questions.
On a standing wave, adjacent nodes sit half a wavelength apart, which is what fixes the harmonic series.
The relation , printed on the AP Physics 2 sheet, is worked in both directions in the wave speed, frequency and wavelength guide.