Sei (清) is the Japanese word for "pure, clear." Sei Shōnagon (c. 966-1017/1025) was the *Heian-era Japanese court lady, contemporary and rival of Murasaki Shikibu, and author of The Pillow Book (Makura no Sōshi) — a collection of observations, lists, anecdotes, and reflections that has shaped Japanese aesthetic sensibility for 1,000 years. "Things that make the heart beat faster." "Things that have lost their power." "Hateful things." Her catalogues of feeling and weather and gesture invented an entire genre — the zuihitsu ("follow-the-brush") — that lives on in modern Japanese prose.*
Featured on UNESCO's Memory of the World register.
Sei 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 Sei reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.