Here are two definitions,

    Critical thinking is the analysis of any attempt at persuasion, based on an evaluation of the form and content of that attempt.  Thinking critically is a strategy for determining how to persuade others, and whether to be persuaded ourselves.  As simple as this sounds, that process of determination can involve literally all we know about ourselves and our world.  { San Jose State University }
    Critical Thinking is essentially applying the principles of Informal Logic to a variety of academic disciplines.  In doing so, we hopefully become aware of the successful patterns of reasoning — successful in that they allow us to move closer to the truth rather than farther away from it.  { Longview Community College }

in addition to the 19 definitions collected by Barbara Fowler.