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]

The Collaborative International Dictionary of English v.0.48:

Water parsnip \Wa"ter pars"nip\ (Bot.)
   Any plant of the aquatic umbelliferous genus Sium,
   poisonous herbs with pinnate or dissected leaves and small
   white flowers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water parsnip
    n 1: stout white-flowered perennial found wild in shallow fresh
         water; northern United States and Asia [syn: water
         parsnip, Sium suave]