original picture; original artwork
Refers to the original physical or digital artwork, as opposed to reproductions, prints, or copies. Common in art and publishing contexts.
Original artwork from a famous picture book is on display at this exhibition.
key frame; key animation
In animation, refers to the main drawings that define the start and end of a movement, drawn by senior animators. Contrasts with 動画 (douga, in-between frames).
In anime production, key frames are drawn first, and then in-between frames are added.
He drew the key frames for that anime.
The manga artist's original artwork is on display at the museum.
She was in charge of the key animation for this production.
原画 refers to the key frames drawn by senior animators that define a movement, while 動画 refers to the in-between frames filled in to complete that movement.
Used because a 原画 (original drawing/artwork) is counted as a flat sheet of paper, like other flat artworks.
Compound of 原 (gen, 'original') and 画 (ga, 'picture'). The animation sense is a specialized extension of the basic meaning.