A "mod" or "modifier" is a term used in various contexts, primarily in gaming and software development. In gaming, mods are user-created alterations or additions to a game that enhance or change gameplay, graphics, or features. They can range from simple tweaks to extensive overhauls, allowing players to customize their experience. In programming, a modifier can refer to keywords that change the behavior of variables or functions. For example, in languages like Java or C++, modifiers such as "public" or "private" define the accessibility of classes and their members. This helps in managing code structure and security.