When talking to a producer/music nerd etc. with a bit of pre-existing DTM knowledge: "It's a vocal VST with (usually) anime mascots that was created/developed in Spain but got popularized and became a huge subculture in Japan."
When talking to a total normie: Something along the lines of "It's a software that makes vocals for music producers.", nowadays I'd add also a little part where I clarify it's ethical and also predated the AI boom by like a decade since people probably will (sadly) mix it up with things like non-consensual SVCs and GANs/diffusion models.
If they like any artists that have history in the Vocaloid scene (for example if they like Chainsaw Man I'd mention Syudou and Kenshi Yonezu made songs for the OST, maybe also Inabacumori since they blew up on TikTok iirc)
If they like VTubers I could mention the parallels between VTer culture and Vocaloid too.
I hate when people overcomplicate others' first introduction to Vocaloid, even weebs themselves might not be as interested in the software or non-Crypton related sides of Vocaloid so there's really no need to mention it until they express interest. I once saw someone say they'd use the term "concatenative synthesis" when introducing someone to Vocaloid completely unironically..
