leadingtone:

You know the fermata: watch the conductor, if any; ponder dinner plans; await sniffles, coughs, and candy wrappers from the audience. But did you know it is one of the most ancient musical symbols, bearing a distinguished pedigree hardly warranting its prosaic ‘birdseye’ nickname?

It was already in use by the time of Dufay, in the early 15th Century. It appears twice here in a phrase from his Ave virgo:

 

The fermata (It. fermare, “to hold”) symbolized in early polyphonic music the end of a completed phrase, where all the polyphonic lines would come together in stable consonance and hold there as one before beginning again. The graphic lineage of a fermata, as Walter de Gruyter has pointed out, is that of a crown: its appearance is in fact identical to the corona symbol of medieval lore, which is itself descended from crown-marks that can be traced back to Assyria and is related to modern astronomical symbols for a solar eclipse. So, then, the fermata in polyphony marked a “crowning moment” in the music, a point of unity. It continued to be used this way through the time of J. S. Bach, in whose chorale settings it appears to mark the ends of phrases without necessarily indicating extended duration.

But other composers of Dufay’s generation—and Dufay himself—also used the fermata to show points at which improvised melisma might occur, and it is from this usage that our modern idea of a fermata as an ambiguously defined pause is derived. This quickly became the symbol’s primary meaning, according to Gehrkens, in the early 18th Century as learned polyphony was supplanted by homophonic textures. 

In the 20th Century, composers began experimenting with fermate of differing shape and size to show relative degrees of duration.