There are lots of ways to define critical thinking. All of them have to do with being logical and rational and with looking for things that are not obvious.
Two such definitions are that critical thinking is A) thinking that focuses on deciding what we should do or what we should believe about certain situations or B) the process of analyzing our own thinking to see if we are thinking in the correct ways. In both cases, we see that we are meant to use logic to look below the surface and get to deeper understandings of situations.
The only difference here is that the first emphasizes only our focus on the issue at hand where the second focuses on critiquing our own though process.