One single horizontal stroke. The most economical possible character — and one of six "indicative" characters (指事字 / zhǐshìzì) where a stroke directly indicates a concept rather than picturing a thing. Identical across 繁體 / 新字体 / 简体.

Despite its visual simplicity, 一 carries a wide semantic load. Beyond "one", it means "first / unified / whole / same / once". 第一 (dìyī, first), 统一 (tǒngyī, unified), 一样 (yīyàng, same), 一起 (yīqǐ, together), 一切 (yīqiè, everything), 一边 (yībiān, one side / while). The expansion from "one" to "everything" passes through "unity".

Mandarin: yī, level 1st tone in citation form — but 一 has the unusual property of tone-sandhi: it shifts to falling 4th tone (yì) before 1st/2nd/3rd tone syllables, and to rising 2nd tone (yí) before 4th tone syllables, but stays 1st tone when ordinal or alone. So 一个 reads yīgè in counting but yìgè in ordinary speech. This is one of the first "advanced" pronunciation rules Mandarin learners must internalize.

Japanese: on-readings イチ (ichi) for counting and ordinals — 一月 (ichigatsu, January), 一人 (one person... but read ひとり / hitori — irregular!), 第一 (daiichi, first); イツ (itsu) in 統一 (tōitsu, unification), 同一 (dōitsu, identical). Kun-reading ひと (hito) survives only in compounds like 一つ (hitotsu, one item) and 一人 (hitori, one person) — both irregular but high-frequency.

Writing tip: in formal/financial contexts, 一 is often replaced by 壹 (the formal banker's form) on checks and contracts to prevent forgery — adding strokes to a horizontal line to read "two" or "three" is too easy.

Memory aid: a single line. The picture is the meaning.