Picture a thermal camera picture of a wall, a bowl of soup, or a whole country. Almost all of it is a cool, even colour. Then one small patch glows. Nothing else has changed — only that patch is carrying more of something than the space around it. Every use of hot spot can be read off that picture: pick the patch, then decide what the 'heat' is. Sometimes it really is heat. Sometimes it is people, or trouble, or a signal, or a screen that answers you back.
可以想成一張熱感應照片：整面牆、整碗湯、整張地圖，大部分是一片冷色，只有一小塊亮起來。hot 是「多」，spot 是「一小塊地方」，合起來就是「這一點特別多」。後面所有用法，都是換掉「多的是什麼」而已：有時候多的是溫度，有時候是人潮，有時候是麻煩，有時候是訊號。
From the word’s history
Wiktionary records the compound as simply hot + spot.
來源只說這個字是 hot（熱）加 spot（一小塊地方）組成的複合詞；其他各種意思之間的連結，是理解上的比喻，不代表歷史上的先後。
Wiktionary, "hot spot"
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