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one; 1
best
first; foremost
beginning; start
a (single); one (of many)
ace
bottom string (on a shamisen, etc.)
「一」 has other readings with different meanings.
一から全部やり直そう。
Let's redo everything from scratch.
一番大切なのは健康だ。
The most important thing is health.
世界に一つだけのプレゼントを作りたい。
I want to make a one-of-a-kind present in the world.