moment's leisure; free minute
Refers to a very short period of free time, often used in expressions like 寸暇を惜しむ (to make the most of every spare moment).
彼は寸暇を惜しんで勉強した。
He studied hard, making the most of every spare moment.
It was such a busy day that I didn't have a moment's free time.
寸暇を利用して散歩する。
I take a walk whenever I have a free minute.
彼は寸暇を見つけて本を読んでいる。
He finds spare moments here and there to read books.
I used a spare moment to check my email.
余暇 refers to leisure time in general (like evenings or weekends), whereas 寸暇 emphasizes a tiny, momentary gap of free time.
Compound of 寸 (a very small unit of length, metaphorically 'a little') and 暇 (free time, leisure).