Great question — answering as the UphillTech team:
It's commonly called a "loudspeaker plug" (or "speaker plug"). As there are other types of speaker plug, "speaker plug 2 pin" will get you plenty of results. The one in your hand is male, the mating part might be called a female plug, or a socket. They're fairly common for low voltage Christmas lights, but the actual voltage and whether AC or DC depends on the lights. I have successfully swapped power supplies between lights, but they were labelled
Full answer by Chris H: https://diy.stackexchange.com/a/310598 (CC BY-SA 4.0).
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