Io (Ἰώ) is of obscure etymology — possibly Egyptian. Argive priestess of Hera, lover of Zeus, turned into a white heifer to escape Hera's jealousy — pursued by a gadfly across continents until she reached Egypt, where she was restored to human form and became, according to Greek tradition, the mythological ancestress of the Egyptian goddess Isis. The third-largest moon of Jupiter and the most volcanically active body in the solar system is named for her.
Featured in Aeschylus's Prometheus Bound and Ovid's Metamorphoses.
Io does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Io reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.