noun

• (historical) One of the four humors making up the body in ancient and mediaeval medicine; said to be cold and moist, and often identified with mucus.
  "Each person's unique mixture of these substances determines his temperament: a predominance of blood gives a sanguine temperament; a predominance of phlegm makes one phlegmatic; yellow bile, bilious (or choleric); and black bile, melancholic."
• (historical, chemistry, alchemy) A watery distillation, especially one obtained from plant matter; an aqueous solution.
  "The attempts made to analyse vegetable substances previous to 1720, merely produced their resolution into the supposed elements of the chemists of those days, namely, salts, Earths, phlegm, and sulphur."