An achievement. This phrase is usually used in reference to a man; however, nowadays, just about anything goes, so one may hear it used occasionally in reference to a female. Often times, the achievement tends to be selfish and/or refers to something sexual (though not necessarily).
1. "He does not genuinely love and care about the girl he just had sex with, to him, she is just another notch in his belt."
2. "He is not in love with me; I do not want to have sex with him and be just another notch in his belt."
3. "Do not have sex with him. He might care about you, but at best you two are just friends- he is not in-love with you; do you want to be just another notch in his belt?"
4. "That girl over there- tonight... she will be the next notch in my belt, and then I will move onto the next one."
5. "I am not ready to settle down. I just want to get as many notches on my belt as I can."