Yes. Assuming that the egg has been fertilized by a sperm, the (naturally) irreversible process of development of a new human has begun. So the day after pill kills that nascent developing life, hence aborts it. It may be too small to see with the naked eye and not have any recognizable human characteristics, but give time and being left alone, it is a person. The question is when does a baby get a soul and become a person. God says it is at the point of conception.