Definition? What definition? There is no definition. Someone told you something in confidence and you keep it confidential if you want to be considered a dignified, trustworthy person of integrity. Unless it's something illegal, egregiously immoral/unethical or they're obviously struggling with things and pose an immediate threat to themself or someone else. It all largely depends on the situation at hand.
In the workplace, ALWAYS heir on the side of caution and assume that everything someone tells you was spoken in confidence. For example, if a coworker tells you they're leaving their job and seeking your advice you don't gossip about it or mention it to anyone. Just be honored that they trusted you enough to tell you. Don't destroy your own professionalism and other people's perception of you by running at the mouth.
However, if someone is contemplating suicide you shouldn't keep that a secret. Of course, that's a sensitive subject that requires tact and paying attention to what they're not telling you. Always ask them if they'd be willing to sit down and meet with so-and-so before just telling so-and-so about it otherwise it will likely be perceived as a betrayal. They're obviously already hurting. Be supportive of your coworkers. Stop competing for a promotion / pay raise and throwing them under the bus like a desperate person for a minute. Be compassionate. Don't alienate them further.