In the context of social gatherings, what term refers to an intruder or unwanted participant?

The term that refers to an intruder or unwanted participant in social gatherings is "interloper." This word captures the essence of someone who enters a situation or place where they do not belong, often disrupting the dynamics of the event or causing discomfort among those present. In social contexts, interlopers can create tension as they violate the established boundaries of a gathering, making it challenging for the original participants to feel at ease.

In contrast, the other terms provided do not convey the same meaning. A flatterer might refer to someone who compliments others excessively, often with ulterior motives, but does not imply an intruder. Callow describes someone who is inexperienced or immature and does not directly relate to the idea of an unwanted participant. Surmisal pertains to the act of guessing or speculating, which is also unrelated to the concept of intrusion in social settings. Thus, "interloper" is the most fitting word for identifying someone who disrupts a social gathering by their mere presence.