一旦
yīdàn · HSK 5
once
一 (one) + 旦 (dawn) = once at daybreak, i.e. once/whenever
旦 shows the sun 日 rising over the horizon 一, meaning dawn. Combined with 一 (one), 一旦 literally reads as one dawn — used for once or some time or other when a condition is met.
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'