一瞬间
yī shùn jiān · HSK 7
split second
⚡ 一 + 瞬 + 间 = one + blink + gap = the span of a single eye-blink
瞬 means to blink. 一瞬间 is literally the gap (间) of one (一) blink — about as long as an eyelid takes to drop and lift, hence a split second.
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.'
间 — gate 門 + sun 日 (originally moon 月)
Originally written 閒, showing the moon 月 shining through a gate 門 — the gap or space between the doors. The moon was later replaced by 日 (sun), giving the modern 间. The core meaning 'space/interval between' extended to time intervals, rooms, and the measure word for buildings.