There are so many aspects of professionalism with regard to teaching.
There is professionalism in terms of being prepared in terms of the knowledge needed to teach in a given content area. A professional teacher has the knowledge that is required for any given lesson. If a teacher does not have that knowledge, he or she makes sure to obtain it before trying to teach the lesson.
There is professionalism in terms of being prepared from a pedagogical point of view. A professional teacher does not simply come into class hoping to "wing it." He or she must always plan lessons carefully and with some thought as to what to do in various situations that might arise within the lesson.
Finally, there is professionalism in terms of personal conduct. A professional teacher comes to work looking like a professional. Such a teacher also conducts him or herself in a manner that will communicate to the students that the teacher is serious about the job of educating them.