The fanlight window is perhaps the most recognisable element of Georgian architecture. The concept of regimented flat-fronted terraces with solid front doors and narrow hallways had presented a problem - how to get light into the entrance hall; this was the elegant solution. It persisted into Victorian times.
And they were elegantly made. Thin glazing bars enabled maximum light through. The framing of multiple small panes kept the cost down - this was the era before sheet glass (that came in around 1832) so large panes would have been extremely expensive or impossible. And, in an era before street numbering was invented, the builder-speculators and their customers took the opportunity to be inventive with the design of fanlights - they tended to differ - in order houses could be more readily distinguished from each other.
John Sambrook made a study of the Georgian fanlight - in a lifetime of restoring them. This simple example is of the "teardrop" category - the "batswing" and "umbrella" being other designs. See: John Sambrook "Fanlights", Chatto & Windus, 1989.