phlogiston
noun
phlo·gis·ton flō-ˈji-stən 
: the hypothetical principle of fire regarded formerly as a material substance
Example Sentences
Recent Examples on the Web Lavoisier didn’t think that Priestley was right about phlogiston, prompting a long-running battle. 
Kat Eschner, Smithsonian, 25 Aug. 2017
Kat Eschner, Smithsonian, 25 Aug. 2017 Word History
Etymology
New Latin, from Greek, neuter of phlogistos inflammable, from phlogizein to set on fire, from phlog-, phlox flame, from phlegein
First Known Use
1733, in the meaning defined above