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]

WordNet (r) 3.0 (2006):

hackmatack
    n 1: poplar of northeastern North America with broad heart-
         shaped leaves [syn: balsam poplar, hackmatack,
         tacamahac, Populus balsamifera]