masticin

from The Century Dictionary.

• noun A substance (C₄₀H₃₁O₂) which remains undissolved on dissolving mastic in alcohol. It amounts to about a tenth of the mastic employed, and has while moist all the characters of caoutchouc, but becomes brittle when dried.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) A white, amorphous, tenacious substance resembling caoutchouc, and obtained as an insoluble residue of mastic.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun organic chemistry A white amorphous substance resembling rubber, obtained as an insoluble residue of mastic.