Interference of light waves produces fringes. In a double slit experiment Young demonstrated that.
When two coherent spherical waves of the same frequency separated by a small distance interfere in space, bands of high intensity and no intensity (darkness) are formed. This phenomenon is due to interference of the waves due to path difference. Where the path difference is an odd multiple of 1/2 wavelength (λ) dark bands are formed. Where the path difference is an even multiple of 1/2 wavelength, bright regions are formed due to constructive interference.
These bands appear with a uniform width and alternately on the screen. The distance between successive bright regions or between successive dark regions is called the fringe width.
Fringe pattern is the display of the alternate bright and dark regions (bands) of equal width.
In Young's double slit experiment fringe width = β = λ D /d , where D is the distance between the slits and the screen and d is the distance between the slits (sources of light).