Iseult is from the Old French Iseut — adapted from the Welsh Esyllt (of fair aspect). Iseult is the Irish princess at the heart of Tristan and Iseult — one of the love legends of medieval Europe, predating and influencing the Arthurian Lancelot-Guinevere romance.
Iseult 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.
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In Pythagorean numerology the letters of Iseult reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.