Thematic coherence means that a game’s mechanics, visuals, sound, narrative framing, and environmental cues all reinforce the same underlying idea, mood, or meaning. When a game is thematically coherent, the player receives one clear impression of what the experience is about. When those layers conflict, the result is tonal confusion or unintended comedy. (CRE341 Lectures, see source-cre341-lectures)
The term is closely related to environmental-storytelling and narrative-design, because coherent themes are usually communicated through repeated motifs across multiple systems rather than through text alone.
Related: environmental-storytelling, narrative-design, game-feel