Pentamethylene \Pen`ta*meth"yl*ene\, n. [Penta- + methylene.]

(Chem.) A hypothetical hydrocarbon, C5H10, metameric with the amylenes, and the nucleus of a large number of derivatives; -- so named because regarded as composed of five methylene residues. Cf. Trimethylene, and Tetramethylene.

Source: Webster's Revised Unabridged Dictionary (1913)