When is a zygote no longer a zygote?

its like this zygote->embryo->foetus->child! it stays as a zygote for only a few days

A zygote is a single cell resulting from fertilization... once the cell undergoes cell division (usually mitosis), then is in an embryo.

In response:  Sorry for my slightly simplified answer regarding the term embryo.  Technically, blastulation (the process that forms the blastocoel within the blastocyte) is occurring in an embryo.  This stated technicality of implantation does not uniquely define zygote vs. embryo.

Zygotes are simply the single cell resulting from syngamy.  Once cell division occurs, it is no longer a zygote. Period.

Sorry, a zygote is a zygote in humans until implantation.  Only bible thumpers think a zygote is a baby.

A Zygote becomes a Blastocyst before it becomes an Embryo.