essential : Idioms & Phrases

• noun an amino acid that is required by animals but that they cannot synthesize; must be supplied in the diet
  

WordNet

• (Biol.), the prominent characteristics which serve to distinguish one genus, species, etc., from another.

Webster 1913

• noun a prerequisite
  sine qua non.
  

WordNet

• (Med.), one that is not dependent on another.

Webster 1913

• noun persistent and pathological high blood pressure for which no specific cause can be found
  hyperpiesis; hyperpiesia.
  

WordNet

• noun an oil having the odor or flavor of the plant from which it comes; used in perfume and flavorings
  essential oil; volatile oil.
  

WordNet

• noun an oil having the odor or flavor of the plant from which it comes; used in perfume and flavorings
  essential oil; volatile oil.
  

WordNet

• (Chem.), a class of volatile oils, extracted from plants, fruits, or flowers, having each its characteristic odor, and hot burning taste. They are used in essences, perfumery, etc., and include many varieties of compounds; as lemon oil is a terpene, oil of bitter almonds an aldehyde, oil of wintergreen an ethereal salt, etc.; called also volatile oils in distinction from the fixed or nonvolatile.

Webster 1913

• noun the primary form of thrombocytopenia (rather than a shortage of platelets caused by other conditions such as tuberculosis or chemical suppression of bone marrow etc.)
  

WordNet

• noun tremor of unknown cause (usually of the hands and head) that develops in older people; often mistaken for Parkinsonism but is not life-threatening and can usually be kept under control
  

WordNet