The Collaborative International Dictionary of English v.0.48:

Frankincense \Frank"in*cense\, n. [OF. franc free, pure + encens
   incense.]
   A fragrant, aromatic resin, or gum resin, burned as an
   incense in religious rites or for medicinal fumigation. The
   best kinds now come from East Indian trees, of the genus
   Boswellia; a commoner sort, from the Norway spruce (Abies
   excelsa) and other coniferous trees. The frankincense of the
   ancient Jews is still unidentified.
   [1913 Webster]

WordNet (r) 3.0 (2006):

frankincense
    n 1: an aromatic gum resin obtained from various Arabian or East
         African trees; formerly valued for worship and for
         embalming and fumigation [syn: frankincense, olibanum,
         gum olibanum, thus]