A compiled program, or an interpreter running some code, is said
to evaluate the expressions it comes across. Once the
instruction has been parsed and the expressions figured out,
it's time to calculate their value. Thus, "a+5" might evaluate
to 6.7 (if a == 1.7), and "len('a'*5)" to 5 (in Python, the
expression 'a'*5 evaluates to 'aaaaa', and len is a function
returning objects' lengths). In many interpreted languages, the
interpreter's own evaluation command is available to the
programmer (or one very much like it); expressions are passed as
strings, and the value is returned. This function is frequently
named eval.  Interpreters are also sometimes said loosely to
"evaluate" code (entire instructions, rather than just
expressions), but strictly speaking, this is execution (of which
expression evaluation is just one part).