IPA: /sɪˈnɛrɪst/
KK: /sɪˈnɛrɪst/
A person who writes screenplays for films or television shows.
The scenarist worked late into the night to finish the script for the new movie.
Scenarist is formed from "scena" (from Latin, meaning scene or stage) and "-ist" (meaning one who practices or is concerned with). A scenarist is someone who creates or writes scenes, particularly in the context of film or theater.
Think of a 'scene' ('scena') that someone is 'creating' or 'writing' — that's what a scenarist does.