plasmid (n.)

1712, "form, shape" (a sense now obsolete), a more classical form of earlier plasm; from Late Latin plasma, from Greek plasma "something molded or created," hence "image, figure; counterfeit, forgery; formed style, affectation," from plassein "to mold," originally "to spread thin," from PIE *plath-yein, from root *pele- (2) "flat; to spread."

Sense of "the liquid part of blood, etc., as distinguished from the corpuscles" is from 1845. In physics, the sense of "ionized gas" is by 1928.