Adelaide is the English form of the Germanic Adalheidis — composed of adal (noble) and heid (kind, sort).
Adelaide is the capital of South Australia, named after Queen Adelaide. The name has been continuously used in English but never enormously popular. Today it sits in the U.S. top 250 and is climbing fast.
Adelaide 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 Adelaide reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.