The Collaborative International Dictionary of English v.0.48:

Hyssop \Hys"sop\, n. [OE. hysope, ysope, OF. ysope, F. hysope,
   hyssope, L. hysopum, hyssopum, hyssopus, Gr. ?, ?, an
   aromatic plant, fr. Heb. [=e]sov.]
   A plant (Hyssopus officinalis). The leaves have an aromatic
   smell, and a warm, pungent taste.
   [1913 Webster]

   Note: The hyssop of Scripture is supposed to be a species of
         caper (Capparis spinosa), but probably the name was
         used for several different plants.
         [1913 Webster]

WordNet (r) 3.0 (2006):

hyssop
    n 1: a European mint with aromatic and pungent leaves used in
         perfumery and as a seasoning in cookery; often cultivated
         as a remedy for bruises; yields hyssop oil [syn: hyssop,
         Hyssopus officinalis]
    2: bitter leaves used sparingly in salads; dried flowers used in
       soups and tisanes

Easton's 1897 Bible Dictionary:

Hyssop
   (Heb. 'ezob; LXX. hyssopos), first mentioned in Ex. 12:22 in
   connection with the institution of the Passover. We find it
   afterwards mentioned in Lev. 14:4, 6, 52; Num. 19:6, 18; Heb.
   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 (Matt. 27:48; Mark 15:36.
   Comp. John 19:29).