dynamic scoping

In computer science, a dynamically scoped programming language is one in which the free variables in a procedure are evaluated in the environment at the point of call. For example:

let a = 5 
in let p = proc () a
       a = 10
   in (p)

will return 10 in an interpreter with dynamic scoping, since the variable a is bound to the value 10 when p is called.

Dynamically scoped programs can be extremely difficult to understand. For example, the code fragments

let a = 3
    p = proc () a
in let f = proc (x) (p)
       a = 5
   in (f 2)

and

let a = 3
    p = proc () a
in let f = proc (a) (p)
       a = 5
   in (f 2)

will returns diffent values. (The preceding example was cribbed from Essentials of Programming Languages by Friedman, Wand, and Haynes)

Compare lexical scoping.