Dahlia is the name of the lush, many-petalled flower, which was itself named in honour of Anders Dahl, the eighteenth-century Swedish botanist.
There is also a Hebrew name Dalia, meaning a branch or bough, which shares the sound, so Dahlia sits at a lovely crossroads of botany and language. In either reading it is fresh, floral and a little dramatic.
Dahlia 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 Dahlia reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.