Public display of affection, which is often shortened to PDA, usually refers to things like holding hands, hugging, and kissing in public. What’s interesting, and what I quite like about the term public display of affection is that it describes more than just an observable activity; it describes the intention behind it. There are all sorts of public touching that people in relationships can engage in which aren’t about displaying affection for one another. It’s the intent to make a public show of your feelings that give a public display of affection its weight.
The topic and the actions that can be described as a public display of affection can be contentious for couples, particularly when one person is very comfortable with public touching and the other isn’t (and if you’re in a relationship with more than one other person I imagine the issue of who displays what affection in public with whom can get even more complicated).