There are a number of definitions, but the words from Jean Rose sum it up beautifully for me
"but what is this elusive essence of plant material called essential oil? It is the heart and soul of the plant. It is the essence that deters bugs from eating the plant. It is the fragrant aromatic heart of the plant that attracts bees and pollinating insects. It is the chemical components contained in the tiny plant cells that are liberated during the extraction process" Jean Rose
Lovely isn't it.
These wonderful aromatic essences are extracted from different parts of the plant (leaves, flowers, bark, fruit) through a process of distillation or mechanical expression. More about that in another post.