The Collaborative International Dictionary of English v.0.48:

Parsnip \Pars"nip\ (p[aum]rs"n[i^]p), n. [OE. parsnepe, from a
   French form, fr. L. pastinaca; cf. pastinare to dig up,
   pastinum a kind of dibble; cf. OF. pastenade, pastenaque.]
   (Bot.)
   The aromatic and edible spindle-shaped root of the cultivated
   form of the Pastinaca sativa, a biennial umbelliferous
   plant which is very poisonous in its wild state; also, the
   plant itself.
   [1913 Webster]

   Cow parsnip. See Cow parsnip.

   Meadow parsnip, the European cow parsnip.

   Poison parsnip, the wild stock of the parsnip.

   Water parsnip, any plant of the umbelliferous genus Sium,
      the species of which are poisonous.
      [1913 Webster]

WordNet (r) 3.0 (2006):

parsnip
    n 1: the whitish root of cultivated parsnip
    2: a strong-scented plant cultivated for its edible root [syn:
       parsnip, Pastinaca sativa]
    3: whitish edible root; eaten cooked