Wavefront
Definition
An imaginary surface connecting all points of a wave that are oscillating in step — like the crest line of an ocean wave.
Technical definition
A surface of constant phase. Far from a point source, wavefronts are spherical; very far away they approximate planes (plane waves). Rays are drawn perpendicular to wavefronts, showing the direction of energy travel.
Example
Ripples from a pebble in a pond form circular wavefronts expanding outward — each ring is a set of points all at the same phase of the ripple.
Common confusion
Wavefronts show where the wave is, not how the medium moves. In a transverse water wave, the wavefront moves outward while water molecules mostly bob up and down in place.