The term "unfolding event" describes the occurrence of a protein (or a domain of a protein) unfolding. This can be seen in an unfolding profile as a transition (from one state to another). Unfolding profiles and their transitions are automatically analyzed for inflection points.

So unfolding events and transitions are pretty much the same thing, except that "unfolding event" describes it from the biochemical/biophysical perspective (what's the protein doing?), while "transition" describes it from a mathematical standpoint (what's the data on the chart doing?). Many proteins show one unfolding event (or transition), meaning that the whole protein unfolds in one go. Other proteins show several unfolding events (or transitions). This can happen if the protein has several domains that unfold more or less independently of one another. Each domain can then show as a separate unfolding event or transition. Antibody unfolding profiles, for example, typically show two or three.

Of course, it's possible that not every unfolding event in the sample is visible as a separate transition, for example if two (or more) events happen at very similar temperatures, then the resulting unfolding profile may only show one transition at that temperature. This is the reason why antibodies sometimes show two, and sometimes three transitions.