Definition: '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.' From:  The Foundation for Critical Thinking Defining Critical Thinking
However, very important to note that knowledge is an essential component of Critical Thinking: 
You cannot think critically about things you know nothing about.  
Note the following:
'Learning critical thinking skills can only take a student so far. Critical thinking depends on knowing relevant content very well and thinking about it, repeatedly.' From: Critical Thinking: Why Is It So Hard to Teach? | Reading Rockets
Worth noting the following references:

Picture shows the British Library Reading Room