one (long cylindrical object) or one item counted with the counter 本; also used to count things like one film, one goal, one phone call, etc.
ビールを一本ください。
Please give me one beer (one bottle).
昨夜、映画を一本見た。
I watched one movie last night.
先制点は監督の采配で一本取った試合だった。
The match was decided by the first goal the manager's tactics produced.
彼から電話が一本あった。
I got a phone call from him.
one version; a single edition or cut (referring to a particular version of something)
この映画は国内版と海外版があり、こちらは国内一本だ。
This film has domestic and international versions; this is the domestic one.
ソフトの一本目のリリースで多くのバグが直った。
Many bugs were fixed in the first release of the software.
one book; a particular book (used to refer to a single book in certain contexts)
図書館にその本が一本しかなくて借りられなかった。
The library only had that one copy, so I couldn't borrow it.
彼は研究のために旧版を一本取り寄せた。
He ordered one copy of the old edition for research.
ippon; one full point in martial arts or a decisive move/score
その選手は先に一本を取って試合を優位に進めた。
That competitor scored an ippon first and gained the advantage in the match.
審判は一本と宣告した。
The referee called it an ippon.
an old/traditional term referring to a geisha (usage varies by region/time)
古い小説には『一本』という語で芸者を指す箇所がある。
In old novels there are passages that refer to geisha using the term 'ippon.'
used as a suffix (〜一本で, 〜一本に) to mean focusing on or relying on a single thing (e.g., to put all one's effort into one thing)
彼はその事業一本で勝負するつもりだ。
He intends to put everything into that one business.
試験に一本集中して準備した。
I focused solely on that one exam when preparing.
チームは今季、攻撃一本に戦略を切り替えた。
This season the team switched strategy to focus entirely on offense.