Compounds of 〇

• 〇
  (n) "kanji" zero
• 〇
  (iK) (n) (1) circle (sometimes used for zero)
  (2) 'correct' (when marking)
• 〇〇
  (n) symbol used as a placeholder (either because a number of other words could be used in that position, or because of censorship)
• ストロンチウム九〇
  (n) strontium 90 (Sr-90)