The most common way to say girlfriend is "novia." However, "novia" can imply a serious relationship leading to marriage because "novia" can also be translated as "fiancee" or "bride."
You can always say "mujer," as in "mi mujer." This denotes either a wife or girlfriend. Literally, "mujer" means "woman" and "mi mujer" means "my woman." If you say "mi mujer," the listener will understand that you are talking about a woman with whom you have a relationship.
"Mi enamorada" is used to describe a woman you love. If you are talking about boyfriend and girlfriend, then you can say "enamorados." If you say "la enamorada," it is understood that you are talking about "the girlfriend." This word is probably the closest translation to the English word "girlfriend."
There are dozens of Spanish slang terms for "girlfriend." These slang terms are generally limited in geographic scope, so what is used in Mexico may not be understood in Chile.
