Chances are, you’ve seen a rose window before—they’re everywhere.
Yes, these circular stained glass windows are the hallmark feature of the worlds most notable cathedrals (think Notre Dame, et al), but they’re also found on the many Gothic-Revival style churches that were built across America in the mid-1800s, from big cities to small towns. If you’ve ever been inside a church, or even just driven past one, you’ve probably seen a rose window.
Or perhaps you’re an avid traveler. Most European vacations include at least one visit to a cathedral. They are popular tourist destinations because of their rich history and magnificent architecture, and rose windows are just one of many attractive features.
But rose windows are a lot more than stained glass. Let’s dive in to the history, origin and significance of rose windows. 
Engraving of the north rose at Notre-Dame, Paris, dating to 1245 (above).
According to Painton Cowen in his book “The Rose Window: Splendor and Symbol” any circular window built of stone and stained glass can be considered a rose window.
The most obvious precursor to the rose window is the oculus, a circular opening designed to let in light. This structural feature dates back to Antique Roman architecture. Oculus windows could be found on most stone structures, and they primarily served the utilitarian purpose of providing light in dark buildings. Of course, this was long before electricity!
Oculus at the church of St Gabriel in southwestern France (above). The decoration of floral and natural motifs is highly reminiscent of Roman designs.
As the Gothic architectural style began to emerge in the 12th century, oculus windows became more ornate with tracery more elaborate (think: stone carvings) as the style spread across Europe. As the style developed, the addition of stained glass became prevalent. Thus was the beginning of the rose window.
The earliest example of a rose window is in northwest Spain at San Miguel de Lillio, built in the 840s near Oviedo. The rose window is only a few feet in diameter and has a distinctive two-layer flower pattern with six petals on the inner layer and fourteen on the outer. Yes, with this flower-like design, it's easy to see why they decided to call it a “rose” window. 
The rose window at San Miguel de Lillio (above), though unglazed (no stained glass) is considered to be the first rose window.
How did these windows come to be called rose windows? There are actually several origins. The word “rose” in English, as well as the French, Italian, and Spanish forms of the word can all be traced back to the Latin word “rosa” meaning flower. Another origin is the French word “roue” or “roe” meaning wheel. No wonder so many rose windows look like flowers and wheels!
Engraving of a rose window from Peterborough Cathedral, England (above). Does it look more like a flower or a wheel?
Another early example of a rose window is St Denis Cathedral in France. St Denis is widely considered the first ever Gothic cathedral, pioneering the new architectural style of even more ornate cathedrals, with distinctive features including pointed arches, ribbed vaults, spires, flying buttresses, and of course, rose windows. The Gothic style spread across Europe from the 12th through the 16th century.
The first Gothic cathedral, St Denis Cathedral in St Denis, France was founded in 1135 (above).
With Gothic cathedrals came stained glass windows, and stained glass wasn’t there just to look pretty — it also served a utilitarian purpose.
The rise of Gothic cathedrals during the 12th through 16th centuries led to a rise of Christianity and church attendance. At this point in history the average person was illiterate, and until the invention of the printing press in the 15th century, Bibles were rare to find anyway. No, folks certainly weren’t reading the Bible back in those days. So how were stories from the Bible shared? This is where stained glass windows became so important—images in stained glass depicted stories from the Bible, making these stories accessible to people of all ages and statuses. 
God, in this 16th century window at Hengrave Hall, creates the universe by the Word, portrayed here as a series of concentric circles. (above).
The east rose of St Etienne, Brie-Comte-Robert, showing Christ at the center surrounded by the twelve apostles with traditional attributes, then the labors of the months, symbolizing time (below). 
Indeed, rose windows are so much more than just pretty things to look at. They are rich with history and cultural significance. Just take it from the French architect Eugene Viollet-le-Duc, who experienced “a beautiful terror” in his encounter with the south rose at Notre Dame:
“The cathedral was shrouded in darkness. My gaze was focused on the stained glass in the south rose window through which the rays of the sun passed, sparkling with great subtlety… Suddenly the great organ came to life; for me it was the rose before my eyes that was singing… As I looked I came to believe in my imagination that some of the panes of glass produced the low sounds and others the high ones: I was seized by so beautiful a terror that I had to be taken out.”
Facts and photos from "The Rose Window: Splendor & Symbol" by Painton Cowen.