Joy is the English word — from the Old French joie, from the Latin gaudia, meaning "joys" or "delights." It became a Puritan virtue name in the 17th century, joining Faith, Hope, and Grace as direct word-names given to daughters.
Joy entered the U.S. top 1000 in 1880 and peaked in the 1950s. Today it sits in the U.S. top 350 and is rising — short, direct, with no need for explanation.
Joy reduces to one — the number of singular brightness.