fèng names the gap or seam itself — the result of two surfaces meeting (or failing to). Common compounds:
门缝 (door gap),
牙缝 (gap between teeth),
裂缝 (crack),
缝隙 (chink, small space),
无缝 (seamless). The classic idiom
见缝插针 (literally 'see a gap, stick in a needle') means making use of every available scrap of time or space — fèng for the gap, the verb sense doesn't enter.