Heidi is the German short form of Adelheid — itself the Germanic original of Adelaide and Alice, meaning "noble kind" or "of noble birth." Heidi has been used in Germanic-speaking countries for centuries.
Today it sits in the U.S. top 500.
Heidi 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 Heidi reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.