Hillary is a feminine given name of English origin, derived from the medieval name Hilary, which was used as both a masculine and feminine name during that period. The name Hilary is believed to have originated from the Latin word 'hilarius', meaning cheerful, bright, or happy. In its feminine form, Hillary has been used in England since at least the 12th century. It was also popular among Puritans in the 17th century and experienced a resurgence in popularity during the mid-20th century.
The name Hillary experienced significant popularity fluctuations over time, with a peak of 27 births in 1954 and a decline to as low as 5 births in certain years like 1982, 1983, and 1991. The highest period of consistency was between 1915 and 1929, with the number of births ranging from 12 to 24 annually. However, after 1973, the name's popularity steadily declined, with fewer than 20 births per year on average.