The Collaborative International Dictionary of English v.0.48:

Sea holly \Sea" hol"ly\ (Bot.)
   An evergeen seashore plant (Eryngium maritimum). See
   Eryngium.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Eryngium \E*ryn"gi*um\ ([-e]*r[i^]n"j[i^]*[u^]m), n. [NL., fr.
   Gr. 'hry`ggion, dim. of 'h`ryggos eryngo; cf. L. eryngion,
   erynge.] (Bot.)
   A large genus of umbelliferous plants somewhat like thistles
   in appearance, cosmopolitan in distribution. Eryngium
   maritimum, or sea holly, has been highly esteemed as an
   aphrodisiac, the roots being formerly candied.

   Syn: genus Eryngium.
        [1913 Webster]

WordNet (r) 3.0 (2006):

Eryngium maritimum
    n 1: European evergreen eryngo with twisted spiny leaves
         naturalized on United States east coast; roots formerly
         used as an aphrodisiac [syn: sea holly, sea holm, sea
         eryngium, Eryngium maritimum]