Easton's Bible Dictionary
(Hebrews `ezob; LXX. hyssopos), first mentioned in Exodus 12:22 in connection with the institution of the Passover. We find it afterwards mentioned in Leviticus 14:4, 6, 52; Numbers 19:6, 18; Hebrews 9:19. It is spoken of as a plant "springing out of the wall" (1 Kings 4:33). Many conjectures have been formed as to what this plant really was. Some contend that it was a species of marjoram (origanum), six species of which are found in Palestine. Others with more probability think that it was the caper plant, the Capparis spinosa of Linnaeus. This plant grew in Egypt, in the desert of Sinai, and in Palestine. It was capable of producing a stem three or four feet in length (Matthew 27:48; Mark 15:36. Comp. John 19:29).
Noah Webster's New International Dictionary of the English Language
(n.) A plant (Hyssopus officinalis). The leaves have an aromatic smell, and a warm, pungent taste.