Rayonnant is a term used to a period in the French Gothic architectural style circa 1240-1350. Following from High Gothic, Rayonnant buildings took the ideas underpinning the French Gothic movement to their most accomplished level. In other schemes of the history of Gothic, Rayonnant comes after "lancet Gothic". After about 1350, the Late Gothic or "Flamboyant" style followed.
Gothic architecture is characterized by light and Rayonnant takes this to the extreme with buildings being so transparent that they appear lace-like from the exterior. The viewer can see through the walls of the building at many different perspectives. A famous example of Rayonnant architecture is La Sainte-Chapelle in Paris. This chapel was built for Louis IX's personal use, and while the lower story is quite enclosed, the second story is almost entirely glazed.
Some sources derive the term from the chapels spreading from the apse, that are typical of the style; others from the tracery of rose windows, also typical of Gothic churches, especially cathedrals. In ideal gothic aesthetics, the petals of the rose radiate from the center of the window, thus the term "rayonnant" (from the French word meaning "to radiate").