Formal or literary numeric 'one', often used in counting or in fixed expressions. In everyday counting, いち is more common.
See also: 一つ (ひとつ)
Count one, two, three.
Used in the adverbial phrase 一に to mean 'solely' or 'entirely', often in formal or literary contexts. Appears in expressions like 一に~による (it all depends on...).
See also: 一に
First, effort; second, effort.
Success depends entirely on effort.
The outcome depends solely on your judgment.