eval is a function which evaluates a string as though it were an expression and returns a result; in others, it executes multiple lines of code as though they had been included instead of the line including the eval.
JavaScript[edit]
In JavaScript, eval is something of a hybrid between an expression evaluator and a statement executor. It returns the result of the last expression evaluated.
Example as an expression evaluator:
foo = 2;
alert(eval('foo + 2'));
Example as a statement executor:
foo = 2;
eval('foo = foo + 2;alert(foo);');
One use of JavaScript's eval is to parse JSON text, perhaps as part of an Ajax framework. However, modern browsers provide JSON.parse as a more secure alternative for this task.
https://en.wikipedia.org/wiki/Eval
Implementation
In interpreted languages, eval is almost always implemented with the same interpreter as normal code. In compiled languages, the same compiler used to compile programs may be embedded in programs using the eval function; separate interpreters are sometimes used, though this results in code duplication.
http://dictionary.sensagent.com/Eval/en-en/