The Collaborative International Dictionary of English v.0.48:

Hackmatack \Hack"ma*tack`\ (h[a^]k"m[.a]*t[a^]k`), n. [Of
   American Indian origin.] (Bot.)
   The American larch (Larix Americana), a coniferous tree
   with slender deciduous leaves; also, its heavy, close-grained
   timber. Called also tamarack.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tamarack \Tam"a*rack\, n. (Bot.)
   (a) The American larch; also, the larch of Oregon and British
       Columbia (Larix occidentalis). See Hackmatack, and
       Larch.
   (b) The black pine (Pinus Murrayana) of Alaska, California,
       etc. It is a small tree with fine-grained wood.
       [1913 Webster]

WordNet (r) 3.0 (2006):

tamarack
    n 1: medium-sized larch of Canada and northern United States
         including Alaska having a broad conic crown and rust-brown
         scaly bark [syn: American larch, tamarack, black
         larch, Larix laricina]