What is a Human Embryo?

Tantalus Prime and I have been discussing abortion. Tantalus takes exception to my observation that human life begins at conception. He believes that the humanity of a human zygote/embryo/fetus isn't a scientific fact, but merely a matter of linguistics:

The exact moment at which a fertilizing egg becomes human... is a horrible scientific question! Is asking the exact size beyond which a stream becomes a river a good scientific question? Of course not, because this is not an empirical problem ...