As someone with three degrees in biology and 20 years as a biologist, I can assure everyone that having a PhD doesn’t make one an expert in anything other than the very narrow topic that they studied (or are studying)–if that. So, I’m inclined to accept Merriam Webster’s definition: “having, involving, or displaying special skill or knowledge derived from training or experience”. Anyone can be an expert in a very limited number of things–it requires nothing but curiosity and motivation.