The numeral '1'. This entry notes the reading 'wan' (ワン), an English-derived pronunciation used in brand names, loanwords, or compounds (e.g., 'one‑two').
彼は背番号１の選手だ。
He is the player with jersey number 1.
ワンツーの掛け声で始めよう。
Let's start on the count of one‑two.
このクラスは学年で１番だ。
This class is number one in the grade.