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In computer programming, a global variable is a variable that does not belong to any subroutine in particular and can therefore can be accessed from any context in a program. They are contrasted with local variables. See also, scope.
They are usually considered bad practice precisely because of their nonlocality: a global variable can potentially be modified from anywhere, and any part of the program may depend on it. A global variable therefore has an unlimited potential for creating mutual dependencies, and adding mutual dependencies increases complexity. See Action at a distance. However, in a few cases, global variables can be suitable for use. They can be used to avoid having to pass frequently-used variables continuously throughout several functions, for example.
Compare this with a constant, which can be seen as a read-only global variable (even though this use of variable is an oxymoron). Because a constant, by virtue of its constancy, can only be "modified" (defined) at one point in the program, all dependencies on a constant are one-way.
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