Chimerism is a genetic trait found in all sorts of animals. It's when you're so close to your fraternal twin you wind up the same organism. Sometimes it does cool stuff like making your eyes different colors or giving you a clearly defined change in hair color at a point. Others get fun little Easter eggs inside our bodies that show up at inconvenient times like people failing the genetic parenting test of someone they gave birth to or extra body parts. It can also cause serious issues if it manifests as mosaicism.

I know nothing about vocaloids so I have no context here, but I can tell you that "chimera" is a word that English borrowed from ancient Greek, which is why it doesn't sound much like a usual English word. There's an old Greek myth about a hero called Bellerophon slaying a monster called the chimera, and since the mythical chimera is a hybrid of several animals we adopted the word as a more general term