Emphasizes that something happens or is done a single time, and no more.
I saw that movie only once.
I'd like to climb Mt. Fuji, even if it's just once.
一回だけ is similar in meaning but uses the counter 回, which is more common in casual counting of actions or events.
Standard kanji spelling.
Numeral variant, common in informal or digital writing.
Ateji spelling using 丈 for だけ; rarely used.
Combination of 一度 (いちど, 'once') and the particle だけ ('only').