“quiet / calm” in Japanese is 静, read sei, jou, shizu (せい, じょう, しず). It is a Grade 4 Kyōiku kanji, taught in year 4 of Japanese elementary school, and appears in words such as 静かな (quiet).
What does 静 mean?
へやが静かだ。
Same sound, different character — the classic trap. Compare them side by side: