Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
fulgē̆trum, i (fem. plur. acc. fulgetras, Plin. 28, 2, 5, § 25.
Post-class. plur. fulgitrua, Hyg. F. 183, unless we should read fulgetra), n. [fulgeo], heat-lightning, lightning: Heraclitus existimat fulgurationem esse velut apud nos incipientium ignium conatus et primum flammam incertam, modo intereuntem modo resurgentem. Haec antiqui fulgetra dicebant, Sen. Q. N. 2, 56, 1; Plin. 2, 54, 55, § 142: si in nube luctetur flatus aut vapor, tonitrua edi; si erumpat ardens, fulmina; si longiore tractu nitatur, fulgetra, id. 2, 43, 43, § 112: quae pingi non possunt, id. 35, 10, 36, § 96.