When a stone is dropped in a still water, waves spread out along the surface of water in all directions with same velocity.
Every particle on the surface vibrates. At any instant, a photograph of the surface of water would show circular rings on which the disturbance is maximum. It is clear that all the particles on such a circle are vibrating in phase, because these particles are at the same distance from the source. Such a surface which envelopes the particles that are in the same state of vibration is known as a wave front. The wave front at any instant is defined as the locus of all the particles of the medium which are in the same state of vibration.
A point source of light at a finite distance in an isotropic medium emits a spherical wave front (Fig: a). A point source of light in an isotropic medium at infinite distance will give rise to plane wavefront (Fig:b). A linear source of light such as a slit illuminated by a lamp, will give rise to cylindrical wavefront (Fig:c).