The outermost expression is enclosed by the [built-in, global scope](./builtins.md), which contains only explicit definitions.
The respective definition types *extend* their enclosing scope by adding new definitions, or replacing existing ones with the same name.
An explicit definition can replace a definition of any type; an implicit definition can only replace another implicit definition.
Each of the above expressions defines which of its subexpressions are enclosed by the extended scope.
In all other cases, the same scope that encloses an expression is the enclosing scope for its subexpressions.
The Nix language is [statically scoped](https://en.wikipedia.org/wiki/Scope_(computer_science)#Lexical_scope);
the value of a variable is determined only by the variable's enclosing scope, and not by the dynamic context in which the variable is evaluated.
> **Note**
>
> Expressions entered into the [Nix REPL](@docroot@/command-ref/new-cli/nix3-repl.md) are enclosed by a scope that can be extended by command line arguments or previous REPL commands.
> These ways of extending scope are not, strictly speaking, part of the Nix language.
