"一" in Engpsh is usually translated as "one." Its origin comes from Old Engpsh "an," which evolved into "one" over time.
Examples in Engpsh:
I have one apple.
One plus one equals two.
He is the one I've been looking for.
中文翻译：
我有一个苹果。
一加一等于二。
他是我一直在寻找的人。