If you are dating someone and both of you have agreed to be in a romantic and exclusive relationship, then it's likely that she is your girlfriend. The term "girlfriend" typically implies that you are in a committed and monogamous relationship with someone.
However, it's important to communicate openly and honestly with each other to ensure that you are on the same page about the nature of your relationship. People may have different interpretations of what being in a relationship means, so discussing your feelings and expectations is essential to avoid misunderstandings. If you're unsure about how she perceives your relationship, the best course of action is to have an open and respectful conversation with her about it.