Wavefront
Also called Wave front
A wavefront is a surface joining points of a wave that are all at the same phase, such as the line running along one crest. It is the bridge between wave language and ray language in geometric optics.
The CED introduces the word in order to define a ray. Essential knowledge 13.1.A.1: a light ray is a straight line that is perpendicular to the wavefront of a light wave and points in the direction of travel of the wave.
That single sentence licenses all of Unit 13. Wavefronts spread out from a source as expanding surfaces; drawing the perpendicular to them at each point replaces the surfaces with arrows, and arrows can be traced through a lens with a ruler. EK 14.4.A.1.ii names the usual simplification: electromagnetic waves are commonly assumed to be plane waves, characterized by planar wave fronts. Far enough from a point source, a small patch of a spherical wavefront is flat to any accuracy you care about.
Where the bridge stops carrying weight. EK 13.1.A.1.i allows rays only where the wave nature of light can be neglected. EK 13.1.A.1.ii is blunt about the limit: rays are not sufficient to understand the spreading of light, and in interference and diffraction the wave nature of the light is important.
So Unit 14 talks wavefronts again. EK 14.7.A.3 says diffraction of multiple wavefronts through a single opening leads to observable interference patterns, and both slit topics state that the amount of interference between two wavefronts depends on the path length difference between them (14.7.A.4.ii, 14.8.A.1.iii). Path length difference is a statement about phase, which is a statement about wavefronts. A ray has no phase to compare.