Again, rubbish.
The definition they brought up made it clear.
Anyway, the term is subjective. In this particular usage we are talking about what subject? The zygote. According to the definition provided from the biology text book quoted, when the cell began to grow was when conception began, otherwise it was a "residual body" that never held life. That definition labeled this point as a "viable zygote".
Now had we been talking about fetus, viability would likely be at the point of implantation in the uterus. And if we are talking of infants we might reach what you are trying to pretend they meant earlier, after you hypocritically remove all context. And even after, in this very thread, you argued that others would remove the context of your posts.
Hypocrite.