late 14c., demonstracioun, "proof that something is true," by reasoning or logical deduction or practical experiment, from Old French demonstration (14c.) and directly from Latin demonstrationem (nominative demonstratio), noun of action from past-participle stem of demonstrare "to point out, indicate, demonstrate," figuratively, "to prove, establish," from de- "entirely" (see de-) + monstrare "to point out, reveal show," which is related to monstrum "divine omen, wonder" (source of monster). Both are derivatives of monere "to remind, bring to (one's) recollection, tell (of); admonish, advise, warn, instruct, teach," from PIE *moneie- "to make think of, remind," a suffixed (causative) form of the root *men- (1) "to think."
Sense of "exhibition and explanation of practical operations" is by 1807. Meaning "public show of feeling by a number of persons in support of some political or social cause," at first usually involving a mass meeting and a procession, is from 1839. Related: Demonstrational.