Definitions I like:
[W]e need to think because the world we live in, however well we learn to cope with it, is constantly forcing us to choose. When experience surprises or disturbs us, we have to “make up our minds,†and, as the phrase suggests, when we do that, not only are we deciding what to do with the world about us; we are deciding what we are or want to be. —Monroe C. Beardsley, Practical Logic (Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1950), x-xi.
[There are] two distinctly different kinds of thinking, creative thinking and critical thinking. Creative thinking may be defined as the formulation of possible solutions to a problem or explanations of a phenomenon, and critical thinking as the testing and evaluation of these solutions or explanations.  –W. Edgar Moore, Creative and Critical Thinking (Boston: Houghton Mifflin Company, 1967) 2, 3.
Critical thinking is the intellectually disciplined process of actively and skillfully conceptualizing, applying, analyzing, synthesizing, and/or evaluating information gathered from, or generated by, observation, experience, reflection, reasoning, or communication, as a guide to belief and action. –Michael Scriven and Richard Paul, “Defining Critical Thinking: A Statement for the National Council for Excellence in Critical Thinking Instruction.â€Â http://www.criticalthinking.org/aboutCT/definingCT.shtml (16 May 2005).
from Daryl Close’s “Teaching Critical Thinking”