Question from a non-physicist: In this context, what does it mean for something to be a "particle"? I.e., if I see some feature in the LHS data, what makes me

Question from a non-physicist: In this context, what does it mean for something to be a "particle"? I.e., if I see some feature in the LHS data, what makes me call it a "particle" vs. some other concept / object / phenomenon?