What is the X in X-mas?
transparent plastic made from nitro-celluloses and camphor, 1871, trademark name (reg. U.S.), a hybrid coined by U.S. inventor John Wesley Hyatt (1837-1900) from cellulose + Greek-based suffix -oid. Used figuratively for "motion pictures" from 1934. Abbreviated form cell "sheet of celluloid" is from 1933 (cf. cel).