One definition might be a commitment to performing at the highest level, to give your best at all times for what you are paid to do. Although that is correct, there are many other facets to being a true professional other than getting paid for doing something.
Most professionals have specialized skills that required independent erudition and effort on their part to attain. They engage in a process of constant evaluation and improvement. A professional makes decisions based on their dedication to the craft and not the current circumstance.
Amateurs are capable of doing some things well under the right conditions, but a professional, as a matter of course, does it well regardless of the situation. A professional is passionate, motivated, and punctual. A professional respects the respectable, but admires the inspirational. A professional is a seeker of knowledge but also a teacher. And the characteristic that separates the professional from the dilettante is an uncompromising commitment to excellence – doing what is required to get the job done at its highest level, even when it is inconvenient.
I have offended God and mankind because my work did not reach the quality it should have.
~~ Leonardo da Vinci, artist, d. 1519, last words