Now, we would like the :term:`operator` to remember not to forget the http protocol, that is to put `http://` at the beginning of our `web_address` variable's value. What if he has forgotten the http protocol? We can add a reminder in case he has forgotten.
We call it a validator.
..glossary::
validator
A validator is a jinja code that parses a variable's value and checks
that this value corresponds to some stated criteria.
This is a `validator` parameter of our variable, and another `jinja` sub-parameter
contains validation code for the variable's value.
