接在產品上的另一件東西
同樣是一個主體加一塊，只是這次兩邊都是摸得到的商品。掛上去的那塊本身可以是完整的東西——一顆鏡頭、一個紙匣、一盒擴充包——但它設計出來就是為了配主體用，單獨拿著沒什麼意思。
noun · B2
A product that is made to be used together with another product, and that is usually sold separately.
配套產品
核心畫面
Picture something that is already finished, with one more piece hung on the side of it. The main thing was there first and works on its own; the extra piece arrives later, is usually paid for on its own, and can be taken off again without breaking anything. An extra cover on an insurance policy, an extra drive on a computer, an extra tool inside a program — the shape is the same every time: one main thing, plus one piece.
可以想成一個已經做好的東西，旁邊再掛上一塊。主體先存在，而且自己就能用；多出來的那一塊是後來才接上去的，通常要另外算…更多收起
可以想成一個已經做好的東西，旁邊再掛上一塊。主體先存在，而且自己就能用；多出來的那一塊是後來才接上去的，通常要另外算錢，拆掉之後主體還是完整的。保險多加的那一項、電腦多插的那張卡、瀏覽器多裝的那個功能，形狀都一樣：一個主體，加一塊。一種想法是：遇到 add-on，先找出主體是誰，再看多出來的是什麼，整句話就清楚了。From the word’s history
Wiktionary records add-on as a deverbal noun from the verb 'add on'. The action of adding something on is turned into the thing that gets added, which is why the word always implies that something else was there first.
來源資料記 add-on 是從動詞片語 add on 名詞化而來（deverbal）。「把它加上去」這個動作被變成名詞之後，指的就是被加上去的那一塊。這也是為什麼這個字一定預設有個更早就存在的主體——沒有主體，就沒有東西可以加。
Wiktionary entry 'add-on' (kaikki source_ref 3f6f69a190036ae4:994540172), etymology_text: 'Deverbal from add on.'
From the picture
裝進軟體裡的擴充
一樣是主體加一塊，只是這一塊看不見也摸不到。裝進去之後主程式多了新本事，移除之後主程式又回到原樣——拆得掉這一點，跟核心畫面完全一致。這也是今天一般人最常遇到的 add-on。
noun · B1
A piece of software you put into a program you already use, so that the program can do more — a browser extension, a plug-in, or extra content for a game.
擴充功能
Also in this word
noun · B2
An extra part that is added to a plan, agreement, price or law that already exists.
附加項目