The Collaborative International Dictionary of English v.0.48:

Shamrock \Sham"rock\, n. [L. seamrog, seamar, trefoil, white
   clover, white honeysuckle; akin to Gael. seamrag.] (Bot.)
   A trifoliate plant used as a national emblem by the Irish.
   The legend is that St. Patrick once plucked a leaf of it for
   use in illustrating the doctrine of the trinity.
   [1913 Webster]

   Note: The original plant was probably a kind of wood sorrel
         (Oxalis Acetocella); but now the name is given to the
         white clover (Trifolium repens), and the black medic
         (Medicago lupulina).
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nonesuch \None"such`\, n.
   A person or thing of a sort that there is no other such;
   something extraordinary; a thing that has not its equal. It
   is given as a name to various objects, as to a choice variety
   of apple, a species of medic (Medicago lupulina), a variety
   of pottery clay, etc.
   [1913 Webster] Nonet

The Collaborative International Dictionary of English v.0.48:

Medic \Med"ic\, n. [L. medica, Gr. ? (sc. ?) a kind of clover
   introduced from Media, from ? Median.] (Bot.)
   A leguminous plant of the genus Medicago. The black medic
   is the Medicago lupulina; the purple medic, or lucern, is
   Medicago sativa.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Medicago lupulina
    n 1: prostrate European herb with small yellow flowers and
         curved black pods; naturalized in North America [syn:
         black medick, hop clover, yellow trefoil, nonesuch
         clover, Medicago lupulina]