Joan f. Koine Greek Ἰωάννα from Hebrew יוֹחָנָן 'graced by God'. Fem. of John.
The name of a minor Biblical character later honored as a saint, an apocryphal early medieval female pope, a 13th C queen consort of Wales, a 14th C princess of Wales, a 14th C queen of Naples, a 14th C queen consort of France, a 15th C saint and 'maid of Orléans', a 15th C queen of Naples, a 15th C queen consort of England, a 16th C French saint, a 16th C queen of England, a 16th C queen consort of England, and a 16th C queen of Castile and Leon. Wycliffite Bible (1395): Joone.
This incredibly popular name gave rise to a plethora of variants and diminutive forms in all cultures. Spellings in -o- reflect the original Greek and Hebrew forms of the name. The French vernacular Jehanne influenced the development of spellings in -a- in England.