Several parts combine into one character.

Compound with debated etymology: 一 (one) + 白 (white). The character meant "many / numerous" in the most ancient layer, before being assigned to the specific number 100. To early speakers, 100 felt like "the largest reasonably-countable number" — the threshold beyond which counting becomes "many". Identical across 繁體 / 新字体 / 简体.

Like the smaller numbers, 百 carries a lingering "abundance / variety" sense alongside its precise numerical value. Compounds like 百姓 (common people — "the hundred surnames"), 百花 (every kind of flower), 百貨 (every kind of merchandise) all use 百 to mean "all sorts" rather than literally 100.

Mandarin: bǎi, dipping 3rd tone. 一百 (yībǎi, one hundred), 百分之 (bǎifēn zhī, "X out of 100" → percent: 百分之五十 = 50%), 百万 (bǎiwàn, million — "hundred ten-thousands"), 百货 (bǎihuò, general merchandise / department store), 老百姓 (lǎobǎixìng, "the old hundred surnames" = ordinary people / commoners — central social vocabulary), 百闻不如一见 (bǎiwén bùrú yījiàn, "hearing a hundred times is not as good as seeing once" — classic idiom).

Japanese: TWO on-readings. ヒャク (hyaku) is dominant — 百 (hyaku, 100), 百年 (hyakunen, century), 百貨店 (hyakkaten, department store — note geminate "kk"). ビャク (byaku) appears with rendaku in compounds like 三百 (sanbyaku, 300 — note voicing), 八百 (happyaku, 800 — irregular). Counting 100s in Japanese involves multiple irregular phonetic shifts: 百 hyaku, 二百 nihyaku, 三百 sanbyaku, 六百 roppyaku, 八百 happyaku — pure memorization required.

The Japanese 八百屋 (yaoya, vegetable shop) literally means "eight hundred shop" — referring to the variety of produce sold, not actual quantity.

Memory aid: 一 + 白 = the ancient symbol for "very many" → fixed at 100.