Gary Dorrien has written a massive three volume history of the making of American liberal theology, and I'm currently working my way through the final volume in the series. It's a solid education reading his books. The most recent volume, for example, has an extensive opening chapter on the influence of personalism (personalism was influential in the life and work of MLK Jr. and our current pope, to name a few significant influences).
In any event, here's Dorrien's working definition of liberal theology:
Fundamentally, liberal theology is the idea of a Christian perspective based on reason and experience, not external authority.
It would be interesting to hear comments just on that definition, but to expand:
"Specifically it is defined by its opennes to the verdicts of modern intellectual inquiry, especially historical criticism and the natural sciences; its commentment to the authority of individual reason and experience; its conception of Christianity as an ethical way of life; its advocacy of moral concepts of atonement or reconciliation; and its commitments to make Christianity credible and socially relevant to contemporary people."