.esm

ESM stands for "Elder Scrolls Master," and is the primary data format used by The Elder Scrolls IV: Oblivion. It contains several types of game data, including audio, video, 3D models, textures, and game menus.

ESM files are referenced by the game, but can also be opened and edited using an ESM editor, such as The Elder Scrolls Construction Set. Custom ESM files can be used to "mod" or alter the gameplay of Oblivion.