I define it this: You are a professional if/when you do your job no matter what the personal or professional circumstances.
Is your company going through a massive layoff? Do you have a personal issue with one of your coworkers or your boss? Is the client/customer you are dealing with a complete jerk? A professional rises above those issues and delivers time and again. Now, obviously, no one's perfect… but a pro consistently executes even if the circumstances have "gotten to" him or her.
Personal anecdote time: I found out at one point in my career that the person who was working for me, a junior designer, was making more take-home pay than I was. I was incredibly upset about it, and I was ready to storm into HR's office and threaten to quit. That was the emotional reaction… but it wasn't the professional reaction.
Instead of screaming and throwing things, I spoke to my manager about it, expressed my frustration with the situation (and NOT with the person who hired him to work on my team)… and then followed up with a direct e-mail. The hiring manager who hired the designer called me and profusely apologized, promising to raise my salary to the appropriate level for my experience)
I didn't "take it out" on my direct report, because he was a good negotiator and cut himself a good deal when he was hired – I would have done the same. And because I didn't yell, my professional reputation maintained intact. Even when management didn't follow through on their promises to correct my salary, I maintained professional and did the professional thing… I found another job.