Cai is the Welsh form of Kay, the name of King Arthur's foster brother Sir Kay, and most likely descends from the Roman name Gaius, whose meaning is uncertain but often given as "to rejoice". In Chinese the same spelling can mean "wealth" or "talent", which makes the name feel at home in several cultures. In the United States Cai ranked number 4610 in 2000, peaked at number 1612 in 2022 and stands at number 2113 among boys' names in 2025.