Several parts combine into one character.

短 is a compound ideograph: 矢 (arrow) on the left plus 豆 (a stemmed ritual vessel / by extension a small bean — providing both phonetic and semantic hints) on the right. The composite logic is wonderfully concrete: ancient Chinese measurement used arrows as the standard unit for "short distance" and beans for "very short / minute amount." Combine the two short standards in one character, and you get the abstract sense "short." 短 forms the canonical pair with 長 (long), and the two are nearly always taught and felt together.

Like many opposites, 短 carries metaphorical weight beyond physical length: short → insufficient → flawed. The character spans two semantic axes (long-short / strong-weak) simultaneously.

Korean reading "dan." 長短 (jangdan, "long-and-short" = pros and cons / the rhythm of a piece of music — Korean traditional music explicitly called rhythm 장단), 短點 (danjeom, flaw / shortcoming — same character pattern across CJK), 短期 (dangi, short term), 短命 (danmyeong, short life), 短文 (danmun, short essay), 短期間 (dangigan, short period of time).

Mandarin duǎn, 3rd tone. 短 (duǎn, short), 短信 (duǎnxìn, "short message" = text message — the standard Mandarin word for SMS), 短发 (duǎnfà, short hair), 短期 (duǎnqī, short term), 短处 (duǎnchu, weak point / shortcoming — note Chinese also uses 缺点 (quēdiǎn) more commonly). Modern Chinese keeps 短 active in everyday speech.

Japanese on-reading タン (tan) — 短期 (tanki, short term), 短所 (tansho, shortcoming — same as Korean 단점), 短編 (tanpen, short story / short film), 長短 (chōtan, long and short). Kun-reading みじかい (mijikai, "short") — 短い (mijikai), 短さ (mijikasa, shortness). And the most important Japanese cultural compound: 短歌 (tanka, "short poem") names Japan's defining classical poetic form, the 5-7-5-7-7 syllable verse that has been the mainstream of Japanese poetry since the 7th century. The Manyōshū, Kokinshū, and other imperial anthologies are nearly entirely tanka. The very name "short song" defines the form.

Memory aid: 矢 (arrow — the short standard) plus 豆 (bean — the very-short standard). Two units of shortness combined.