In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other objects. The object that the metaobject is about is called the base object. Some information that a metaobject might store is the base object's type, interface, class, methods, attributes, variables, functions, control structures, etc.

A metaobject protocol (MOP) is an interpreter of the semantics of a program that is open and extensible. Therefore, a MOP determines what a program means and what its behavior is, and it is extensible in that a programmer (or meta-programmer) can alter the program's behavior by extending pieces of the MOP. The MOP exposes some or all internal structure of the interpreter to the programmer. The MOP may manifest as a set of classes and methods that allow a program to inspect the state of the supporting system and alter its behaviour. MOPs are implemented as object-oriented programs where all objects are metaobjects.

MOPs may be runtime or compile time. The metaobjects of runtime MOPs exist while the program itself is executed. The metaobjects of compile-time MOPs, however, exist only when the program is compiled. They may alter or extend the compilation process, but do not exist when the program is running.