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]

Easton's 1897 Bible Dictionary:

Frankincense
   (Heb. lebonah; Gr. libanos, i.e., "white"), an odorous resin
   imported from Arabia (Isa. 60:6; Jer. 6:20), yet also growing in
   Palestine (Cant. 4:14). It was one of the ingredients in the
   perfume of the sanctuary (Ex. 30:34), and was used as an
   accompaniment of the meat-offering (Lev. 2:1, 16; 6:15; 24:7).
   When burnt it emitted a fragrant odour, and hence the incense
   became a symbol of the Divine name (Mal. 1:11; Cant. 1:3) and an
   emblem of prayer (Ps. 141:2; Luke 1:10; Rev. 5:8; 8:3).

     This frankincense, or olibanum, used by the Jews in the temple
   services is not to be confounded with the frankincense of modern
   commerce, which is an exudation of the Norway spruce fir, the
   Pinus abies. It was probably a resin from the Indian tree known
   to botanists by the name of Boswellia serrata or thurifera,
   which grows to the height of forty feet.