What is considered a wavelength?

A sine wave doesn't necessarily have an intrinsic "starting point", you usually can draw its curve starting at any phase and call the corresponding point the beginning of the cycle:

Quoting Wikipedia:

Wavelength λ, can be measured between any two corresponding points on a waveform

and

Wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests (on top), or troughs (on bottom), or corresponding zero crossings as shown.

It is both as both lengths are the same. It doesn't matter which point you start measuring from - as long as you measure it to the same point in the next cycle. Obviously it's easier to choose starting points where you can easily tell where it is in the next cycle $($that is when $y=0$ or at the highest/lowest point in the wave$)$.