What are essential oils?

"Essential oils" have been something that were a part of my life for about 20 years. They were something that my mom introduced me to when I was a lot younger, burning things like peppermint first thing in the morning to wake us up and then using things like Theives oil to ward off sickness. But it wasn't until into my university experience in academia when I studied more microbiology and virology and really tried to learn about how those oils interact with our bodies, that I gained a better understanding of what "essential oils" are.