Actually, it is you abortionists who are refusing to come up with a reasonable definition because you insist on clinging to your lies about the status of unborn humans.
By your standard, there was nothing morally wrong with slavery, as long as the majority agreed that blacks were not human enough to deserve human rights. By this standard it was morally correct to use small pox laden blankets to exterminate native Americans, because the prevailing opinion was that we were not human enough to deserve any rights.
The standard of "what people can agree on" has been used before, mutliple times. History is FULL of instances where the ruling majority used their opinions to dehumanize a targeted group of humans for their own purposes: to enslave, kill, steal their land, etc. etc. etc. Yet every single time opinion has beeen the standard by which human rights are withheld, history has shown that attitude to be morally corrupt.
How is it different this time?
You're convinced of your moral superiority on this issue. You're convinced that abortion is murder.
If you're right, society will catch up. Your "side" repeatedly invokes slavery as an example of when the majority agreed on something immoral. Organized slavery - at least in the U.S. - fell under the sheer weight of its immorality. People caught up very quickly.
I don't see that happening with abortion, but I could be wrong. I'm always amazed when I discuss this with pro-lifers how callous & casual they are about the idea of women being forced to carry to term. I think most people get that.