adverb
closely; compactly; without gaps
Describes things packed tightly together, leaving no empty space. Often used with verbs like 詰める, 並べる, or 敷き詰める.
箱にリンゴを隙間なく詰める。
Pack the apples tightly into the box.
本棚に本が隙間なく並んでいる。
The books are lined up on the shelf with no gaps.
From 隙間 (すきま, 'gap') + なく (adverbial form of ない, 'not'). Literally 'without gaps'.