停下來的是機器或工作
最貼近字面的一層：該運轉的東西停著，那段時間就被算出來。
noun · B2
The time when a computer, machine or system is not working and cannot be used, usually because it has broken, crashed or lost power.
停機時間
核心畫面
Picture a machine that has stopped. The clock keeps running, but nothing is being made. Downtime is that stretch of empty time — the hours when the thing that should be going is not going. What has stopped can be a server, a factory line, a whole business in its quiet month, or a person who has finished what they were doing.
可以想成一台機器停了下來。時鐘還在走，但東西沒有在做。downtime 就是中間空掉的那一段時間——該動的沒有在動。…更多收起
可以想成一台機器停了下來。時鐘還在走，但東西沒有在做。downtime 就是中間空掉的那一段時間——該動的沒有在動。停下來的可以是伺服器、產線，也可以是淡季裡的一整間店，或是手邊事情做完的人自己。From the word’s history
Wiktionary records the word as down (out of order; out of service; inoperable) + time, which is exactly the picture: the time during which something is down.
Wiktionary 記的組成是 down（停擺、不能用）加 time。這正好就是那個畫面：某樣東西 down 著的那段時間。
Wiktionary entry for downtime, etymology_text: From down ("out of order; out of service; inoperable") + time.
Two branches
停下來的是人
把同一個畫面搬到人身上：你手邊的事停了，那段時間就是空的、是你自己的。
noun · B2
Time when you have stopped working or studying and can rest or do whatever you like; a quiet break between busier periods.
休息空檔