A Definition of Critical Thinking could be:

Critical thinking is the ability to analyze and reason through the bits and pieces of seemingly disparate information. Central to critical thinking is deliberate analysis using reasoning and judgement. When one thinks critically, one judges the authenticity, worth, or accuracy of something.

Essentially, critical thinking is evaluative in nature.

Skills central to critical thinking include having the ability to:

-- distinguish between verifiable facts and value claims
-- distinguish from irrelevant information, claims, or reasons
-- determine the factual accuracy of a statement
-- determine the credibility of a source
-- identifiy ambiguous claims or arguments
-- identify unstated assumptions
-- detect bias
-- identify logical fallacies
-- recognize logical inconsistencies in a line of reasoning
-- determine the strength of an argument or claim

References

Norris, S. P. (1985). Synthesis of research on critical thinking. Educational Leadership, 40-45.