Eval
Short for evaluate, eval is a function that can be found in various different interpreted and compiled programming languages that have a variety of different abilities depending on the programming language you're using.
For example, in Perl, PHP, and JavaScript the eval command can be used to interpret a variable string as code and execute it. Below is a basic example of how this could be used in Perl. This could also be extended upon by taking a complete Perl program stored in another file and executing it.
my $one = 1;
my $two = 2;
my $total = 0;
my $example = '$total = $one + $two';
eval $example;
print $total;
In the above example, the string stored in the $example variable is executed using the eval command.
Also see: Programming definitions