Boyd’s Bible Dictionary:

(free burning). The yellowish gum used in sacrificial fumigation (Ex. 30:7-9; Lev. 16:12,13; Rev. 8:3). A mixture of gums and spices (Ex. 30:34-38).

Concise Bible Dictionary:

Frankincense—Boswellia Sacra
A fragrant resin. It was an ingredient in the holy anointing oil, and was used in the temple service (Ex. 30:34; Lev. 2:1, 15-16; Lev. 5:11; Lev. 24:7; Song of Sol. 3: 6, Song of Sol. 4: 6, 14). It formed part of the gifts presented to the Lord by the Magi (Matt. 2:11); and was among the things carried to Babylon the Great (Rev. 18:13). It is traced to the Boswellia serrata of the botanists, which grows in India. By cutting slits in the bark the gum exudes. The best is white and bitter to the taste, though the yellowish in color is extensively used. The Mahometans choose the white, but the Greek and Roman churches use much of the colored.
Frankincense—Boswellia Serrata