身體與機械的縮回
最貼近字面的一支：真的有一截東西被拉回去了。指的是動作本身，所以不可數。
noun · C1
The act of pulling a part of something back in, so that it sits inside the main body instead of sticking out.
縮回
核心畫面
Something that was put out is pulled back in. The claws go back into the paw, the wheels fold back into the plane, and afterwards the surface is smooth again — as if nothing had been sticking out.
retraction 可以想成：伸出去的東西被收了回來。貓爪縮回肉墊裡，起落架收進機腹裡，收完之後表面平平的，看不出…更多收起
retraction 可以想成：伸出去的東西被收了回來。貓爪縮回肉墊裡，起落架收進機腹裡，收完之後表面平平的，看不出剛才有東西伸出來過。主體沒有變小，動的只是伸出去的那一截。英文把這個動作借去講話：說出口的話也像伸出去的手，收回來就是 retraction。From the word’s history
re- 'back' + traction 'pulling' (or retract + -ion). The word literally names a pulling-back.
這個字可以拆成 re-（往回）加上 traction（拉），也可以看成 retract 加 -ion。字面就是「往回拉這件事」，所以實體的縮回是它最貼字面的用法。
Wiktionary entry for retraction (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1410580997:7110): 'From re- + traction or retract + -ion.'
Three branches
說出去的話收回
把伸出去的那一截換成一句話。話一旦說出口就像手伸了出去，公開收回來就是 retraction。這一支是可數的，因為每一次收回都是一份文件、一則聲明。
noun · C1
A public statement saying that something you said, wrote or printed before was not true, and that you are taking it back.
撤回聲明
數學上的收縮
同一個「往回收」的畫面，收的對象變成整個空間。數學家借這個字，是因為它描述的正是把大的壓回小的、而小的原地不動。
noun · C2
In mathematics, a map that sends a whole space onto a smaller part of itself, while leaving every point of that smaller part exactly where it already was.
收縮映射