What does it truly mean to have a girlfriend?

I know that this question may be silly to some of you..but have any of you ever really tried to analyze this? A girl could be doing stuff for you at the office, buy you a gift on Christmas Day and on your birthday, go to a couple of movies with you and maybe have a cup of coffee with you after work, but she still doesn’t have to be your girlfriend. When you have a girlfriend, what does it truly mean? Is it sex that makes it solid? Does she have to live with you? Do you have to not hang out with other girls that you accustom hanging out with?