Objet Petit A is a description of the philosopher Lacan. Babies state of wholeness with the mother he says. When you grow up and leave your mother, no matter what you put in your life, nothing fills that lack, he states. So, one has the desire to replace this feeling with everything. Our whole life goes by replacing that feeling of deficiency with big "A"s. In short, it is an object of desire that will not be filled forever, Objet Petit A.