Norm Geras defines it:

Thinking, now, about the meaning of 'pro-Israel', I'd say that not much more is needed as a basis for describing oneself in this way than some combination of supporting Israel's right to exist, having the interests of the country and its people at heart, liking features of the country and its traditions, taking exception to the special vilification of it common amongst its enemies (both regional and worldwide) and which go beyond normal political criticism, resisting, more generally, the idea that Israel is to be singled out and judged, or treated, according to standards that don't apply to other countries, and so on.

The trouble is: Israel is a rare example of a nation-state that is only a few decades old in a long-inhabited and civilized part of the world, where most of its neighboring states dispute its right to exist at all.

More to the point, overwhelming numbers of the inhabitants of its neighboring states would like to see it disappear from the map altogether. (Of how many other states can that be said?) To add to the complications, Israel isn't even now - geographically - what it was at its inception. Its territory has changed sporadically, making the concept of being "pro-Israel" a somewhat complicated one. And its demography as a predominantly Jewish state is also in flux.

My own definition of pro-Israel would simply be, I think: support for the existence of a secure Jewish state in Palestine. That's my position, and it is as deeply held as it is open to all sorts of arguments about what is best for its security and the interests of the US. I think it should easily be enough to earn one's credentials as a Zionist, as I proudly and passionately remain.