The Collaborative International Dictionary of English v.0.48:

Mayweed \May"weed`\, n. (Bot.)
   (a) A composite plant (Anthemis Cotula), having a strong
       odor; dog's fennel. It is a native of Europe, now common
       by the roadsides in the United States.
   (b) The feverfew.
       [1913 Webster] Mazama

The Collaborative International Dictionary of English v.0.48:

Fennel \Fen"nel\ (f[e^]n"n[e^]l), n. [AS. fenol, finol, from L.
   feniculum, faeniculum, dim. of fenum, faenum, hay: cf. F.
   fenouil. Cf. Fenugreek. Finochio.] (Bot.)
   A perennial plant of the genus F[ae]niculum (F[ae]niculum
   vulgare), having very finely divided leaves. It is
   cultivated in gardens for the agreeable aromatic flavor of
   its seeds.
   [1913 Webster]

         Smell of sweetest fennel.                --Milton.
   [1913 Webster]

         A sprig of fennel was in fact the theological smelling
         bottle of the tender sex.                --S. G.
                                                  Goodrich.
   [1913 Webster]

   Azorean fennel, or Sweet fennel, (F[ae]niculum dulce).
      It is a smaller and stouter plant than the common fennel,
      and is used as a pot herb.

   Dog's fennel (Anthemis Cotula), a foul-smelling European
      weed; -- called also mayweed.

   Fennel flower (Bot.), an herb (Nigella) of the Buttercup
      family, having leaves finely divided, like those of the
      fennel. Nigella Damascena is common in gardens. Nigella
      sativa furnishes the fennel seed, used as a condiment,
      etc., in India. These seeds are the "fitches" mentioned in
      Isaiah (xxviii. 25).

   Fennel water (Med.), the distilled water of fennel seed. It
      is stimulant and carminative.

   Giant fennel (Ferula communis), has stems full of pith,
      which, it is said, were used to carry fire, first, by
      Prometheus.

   Hog's fennel, a European plant (Peucedanum officinale)
      looking something like fennel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Anthemis cotula
    n 1: widespread rank-smelling weed having white-rayed flower
         heads with yellow discs [syn: mayweed, dog fennel,
         stinking mayweed, stinking chamomile, Anthemis
         cotula]