It is what an essential oil?

As its name does not indicate it essential oils do not contain a greasy substance like the vegetable oils obtained with presses (corn, sunflower oil, sweet almonds, etc...). 
It is about natural secretion elaborate by the plant and contained in the cells of the plant, that is to say in the chalice, the stem, the bark or all other part of the plant. 
Essential oil is obtained by the distillation of the plant in alembic.  
According to wished oil, one will take whole or part of a specific plant to extract quintessence, the biochemical structure from it, in order to capture it at medical ends or hygiénique.  
the composition of essential oils is very complex. Terpenes, aldehydes, ketones, phenol, lactones, esters, are components which one finds in essential oils. 
Very volatile essential oils do not grow rancid, are soluble in oil and alcohol but not in water.