A clear, light, easily burned plastic that could be drawn out into a long strip — the strip that moving pictures were printed on. Hold that strip and the rest follows: first the material itself, then the whole of cinema, named after the stuff it used to be carried on.
The name was a company trademark: cellulose plus -oid, 'the stuff like cellulose'. The name says what it is made of, which is why it stayed attached to the strip, and then to the films the strip carried.
這個名字本來是一家公司的商標：cellulose（纖維素）加上 -oid（像…的東西），字面就是「像纖維素做出來的那種材料」。名字直接說出它的成分，所以它牢牢黏在那條膠卷上，後來也跟著黏到膠卷承載的電影上。
Wiktionary entry 'celluloid', etymology_text supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1145345912)
Two branches
材料本身
核心畫面就停在這裡：那條可以拉長、透明、很怕火的塑膠。
noun · C1
A hard, light plastic made mainly from cellulose, used in the past for photographic and cinema film and for small objects such as combs, collars and toy dolls. It catches fire very easily, so safer plastics have replaced it.
賽璐珞
用材料代稱電影
用載體代替它承載的東西，是英文常見的借代。膠卷是電影的身體，久了就拿來指電影本身。這是一條意義上的延伸，不是另一個來源。
noun · C1
If something is on celluloid, it has been filmed, so it is kept and can still be watched as a movie. The phrase stands for cinema as a whole and sounds formal or literary.
留在電影裡