When programs become large, naming conflicts can occur when a function or global variable defined in one file has the same name as a function or global variable in another file. Even just a similarity between function names can cause hard-to-find bugs, since a programmer might type the wrong function name.
The language features that allow this are usually called the module system because programs are broken up into modules that are compiled separately (or loaded separately in an interpreter).
Older languages, like C, have limited support for name space
manipulation and protection. In C a variable or function is public by
default, and can be made local to a module with the
keyword. But you cannot reference public variables and functions from
another module with different names.
More advanced module systems have become a common feature in recently designed languages: ML, Python, Perl, and Modula 3 all allow the renaming of objects from a foreign module, so they will not clutter the global name space.
In addition, Guile offers variables as first-class objects. They can be used for interacting with the module system.