An essential oil is a water-immiscible material produced by distillation from some plant material. The material (flowers, leaves, stems, or roots, depending on the plant) is put in an alembic over water, and the volatile compounds, which require less vapor pressure to evaporate with the presence of steam, distill into a receiving vessel. The upper portion is usually the oil, the lower being the hydrosol. They are drained with two spigots.
Most oils are distilled in a single process. The exception is ylang-ylang, which takes 22 hours to complete distillation. It is distilled fractionally, producing several grades.