1The number one (1). The basic numeral for a single unit or the first in a series, read いち. The variant 壱 (and historically 弌, 壹) is used in legal documents to prevent forgery.
一に二を足すと三になる。
One plus two is three.
彼女は試験で一位になった。
She came in first place in the exam.
数字の一が読めるようになりました。
He has learned to read the digit one.
領収書には「金壱万円」と書かれていた。
The receipt was written 金壱万円 (ten thousand yen), using the legal form of 'one'.
2Best, top — used as a suffix attached to a place or scope to indicate 'the best within ~', as in 日本一 (best in Japan), 世界一 (world's best), 町一 (best in town).
彼女は日本一の歌手と呼ばれている。
She is called the number one singer in Japan.
この眺めは世界一だと思う。
I think this view is the best in the world.
町一の人気店は朝から行列ができている。
The most popular shop in town has a line forming from the morning.
3First; foremost; principal — used in literary or formal expressions of the form 一の~ to mean 'the foremost / principal ~'. A somewhat archaic usage.
彼女は宮中で一の人と称された。
She was hailed as the foremost lady at the imperial court.
4Beginning; start — the very first point of something. Used in expressions like 一から始める ('start from scratch') or 一からやり直す ('do over from the very beginning').
失敗したから、一から出直すしかない。
I failed, so I have no choice but to start over from scratch.
彼は新しい事業を一から立ち上げた。
He built up the new business from scratch.
5A (single); one (of many) — a prefix used before a noun to mean 'one particular ~' or 'a single ~', singling out one instance from a larger set, as in 一意見 ('one opinion'), 一例 ('one example'), 一現象 ('one phenomenon').
一意見として申し上げますが、計画には無理があります。
If I may offer one opinion, the plan has some problems.
一例として、こちらの商品をご紹介します。
As one example, let me introduce this product.
6Ace — the playing card with the value 1, often the strongest (or sometimes weakest) card in a game.
彼はハートの一を引き当てた。
He drew the ace of hearts.
手札に一が二枚あれば、かなり有利だ。
Having two aces in your hand puts you in a very strong position.
7The bottom (lowest-pitched, thickest) string of a shamisen or other three-stringed instrument; also called 一の糸. It is thicker and tenser than the second and third strings.
三味線の一の糸が切れてしまった。
The bottom string of the shamisen has snapped.