The term 'backslider" refers to someone who was once demonstrating their faith though the works of their life but now is no longer doing that.
Simply said, they used to act like a “good” Christian and now that don’t.
I has nothing to do with the condition of their heart or the status of their salvation. In fact, in order to be considered a “backslider”, you must have already turned your life over to God and accepted Jesus as your savior. Once you do that and receive complete forgiveness for all sins (past ,present and future) you have salvation. In theory your outward behavior should then demonstrate the change in your heart. But as well all know, old habits die hard and that is not always the case.
I dislike the term because I feel it is judgemental and we cannot judge the heart of men, only God can do that.