For example, in the following lambda expression, λf . g(f(λx . x),x,y),
the variable x occurs three times. The first occurrence, since it immediately follows a λ, introduces a new “binding” of x, and is therefore called a binding occurrence. The second occurrence of x falls inside the “scope” of this binding and is therefore called a bound occurrence. The third x is not within the scope of any such binding and is therefore called a free occurrence. Equally, the variable f has a binding occurrence and a bound occurrence, while g and y just have one free occurrence each. Since only x, y, and g have free occurrences, they are referred to as the free variables of the expression. The value of the whole expression then depends on what values are given to these free occurrences.
Note that freeness depends on the expression under consideration; thus, although f does not occur free in the whole expression above, it does so in the subexpression g(f(λx . x),x,y).
"free variable." A Dictionary of Computing. . Encyclopedia.com. (October 22, 2018). http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/free-variable
"free variable." A Dictionary of Computing. . Retrieved October 22, 2018 from Encyclopedia.com: http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/free-variable