Zetian (則天) means "emulating heaven" in Mandarin. Wu Zetian (624-705 CE) was the only woman in Chinese history to rule as Emperor in her own right, founding the brief Zhou dynasty.
She remains one of the most consequential rulers of Chinese history.
Zetian 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 Zetian reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.