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The TeX command `\renewcommand` is used to redefine an existing command. This is a powerful feature that allows users to change the behavior or appearance of a pre-defined command, or a command that has been defined by a package. It is distinct from `\newcommand`, which is used to define a new command that does not already exist. When using `\renewcommand`, one must be careful not to redefine fundamental TeX commands unless the implications are fully understood, as this can lead to unexpected and potentially unrecoverable errors.

The syntax for `\renewcommand` is similar to `\newcommand`. It takes the command to be redefined as the first argument and the new definition as the second argument. For example, `\renewcommand{\setminus}{\smallsetminus}` would redefine the backslash symbol used for set difference to be a smaller version if a `\smallsetminus` command were available. If the command takes arguments, these are indicated within the definition using `#1`, `#2`, and so on, just as with `\newcommand`.

The `\renewcommand` command is often used in document customization to adapt existing TeX functionality to specific stylistic requirements or to simplify complex commands. It is a core part of TeX's extensibility, enabling users to tailor the typesetting system to their precise needs. It is important to ensure that the command being redefined actually exists; attempting to redefine a non-existent command will result in an error, much like trying to use a command that hasn't been defined.