My favorite definition of critical thinking is one that I learned from an office mate who was a philosophy professor: It consists of the willingness, in response to any given claim, to ask two questions:

1. What do you mean by [key term]?

2. Why should I believe what you say about [key term]?

The first leads to clarification--consensus on what exactly we are talking about. The second, to justification--the rationale(s) for what we are saying about it.