Well, I never had that one asked of me at an interview, but I thought about it before going into my interview. My answer would have been something along the lines of:
In the realm of optometry, professionalism is concerned with the way in which a doctor presents him/herself and how he/she should interact with his/her patients, as well as other professionals. What this entails is understanding the limits of interaction and making sure that the patient is made to feel comfortable throughout the process of an examination. It also requires that one work with, rather than compete against, other health professionals to provide the best healthcare for his/her patients. Maintaining a decorum of respect and being generally non-jugmental in dealing with patients, are some of the ways in which I personally exhibit professionalism.
That's my short answer. I don't know if you want to expand or add a third facet to it if you think that this would not take up enough "talk time".