noun
Refers to the main song of a movie, TV show, anime, game, or similar production. Often used for both opening and ending themes.
このアニメの主題歌はとても人気がある。
The theme song of this anime is very popular.
映画の主題歌を有名な歌手が歌
A famous singer performs the movie's theme song.
エンディングテーマ specifically refers to the ending theme, whereas 主題歌 can refer to either the opening or ending theme.
Compound of 主題 (theme, subject) and 歌 (song).