the person one dates, makes out with, or sleeps with while waiting or actively searching for the love of one's life - who is quite often found during or immediately after relationship with said placeholder.
Though also viable, cute, and longing for a longterm relationship, there is often some unknown 'thing' in the placeholder's manner, ambition, world view, or timing that often prevents the placeholder from being "the one."
"I just found out Chris married the woman he started dating when he was still sleeping with me. He told me he didn't want a serious relationship. I feel like such a placeholder."