"First, as we step through this program, we don’t think about specific values of x like 13. Instead, we think about the set of possible values x might take on. At different points in the program, x might be an element of all integers, negative integers, or positive integers plus 0. In other words, we think of x as a symbolic value (i.e., a set of possible values) rather than a concrete value (i.e., a particular element of that set of possible values)."

vs:

"We expound a view of type checking as evaluation with ‘abstract values’."

http://okmij.org/ftp/Computation/FLOLAC/lecture.pdf