One way to hold this word: something that should run straight steps inward, and the step stays. A flat surface dips where a thumb pressed; an edge goes in and out where it was cut; a line of writing starts late instead of hard against the margin. In every case you are looking at the same thing — a place where the line moved in, and the gap it left behind.
From the word’s history
From the verb indent plus -ation, which names either the action or what the action leaves behind. That double job is why the word covers both the act of indenting and the dip or gap itself.
由動詞 indent（把一行往裡推，或在邊緣切出缺口）加上 -ation 組成，而 -ation 同時可以指「這個動作」和「動作留下的東西」。這正好解釋了這個字的用法：它既可以指縮排或壓凹這件事，也可以指凹下去的那一塊、縮進去的那段空白。
Wiktionary etymology supplied in this packet: 'From indent + -ation' (source_ref kaikki:3f6f69a190036ae4:648127386:26164)
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