Two horizontal strokes. A direct extension of 一 by adding one more line. Identical across 繁體 / 新字体 / 简体.

Mandarin: èr, falling 4th tone. But here is one of the first counter-intuitive rules of Mandarin: when counting two of something with a measure word, Mandarin replaces 二 with 兩 / 两 (liǎng): 两个人 (liǎng gè rén, two people) — not 二个人. The pattern: use 二 for ordinal numbers and pure counting (二月, February; 第二, second), but 两 (or 兩) when "two of [things]" with a classifier. Japanese and Korean keep 二 in both situations, so this is a Mandarin-specific rule learners must memorize early.

2 also means "ordinal second" without 第 prefix in many Asian contexts: 二院 (the Senate / second house), 二流 (second-rate), 二战 (èrzhàn, World War II — abbreviation of "Second World War").

Japanese: on-readings ニ (ni) for everyday counting — 二月 (nigatsu, February), 二人 (futari... wait, irregular!), 二日 (futsuka, second day of month — irregular kun-reading); ジ (ji) in formal compounds like 二次会 (nijikai, after-party — "second meeting"). The kun-reading ふた (futa) shows up in 二つ (futatsu, "two items") and 二人 (futari, "two people") — these are high-frequency irregular readings.

Financial form: 弐 (also written 貳) replaces 二 on Japanese checks and Chinese contracts to prevent forgery (you cannot easily turn 弐 into 参 the way you could turn 二 into 三).

Memory aid: two lines, top shorter than bottom (Chinese convention) or equal length (Japanese convention).