The term comes from Greek Mythology, and refers to an animal comprised of portions of different animals.
Regarding plants, I have always understood it to be a "mutation" that is visually manifested, and that often is unstable. Panache/Panachee is often referred to as a chimera, and if you have a tree for a while, portions of it often revert back to the original form (green). I have called it "Reverse" and it differs from Panache by one gene, acc. to the DNA work done at USDA.
You can Google it and you will get way more than you want to know, with regard to genetics, animals, plants, people, etc.