A person who takes part in a public demonstration or protest; one who demonstrates.
From demonstrate + -ant (a suffix forming nouns indicating an agent or person who does something). Related to demonstrator, this Latin-influenced form emphasizes the active role of the protester.
Demonstrant sounds fancier and more formal than 'protester,' which is why governments and formal reports use it—it's linguistic code for treating protesters as serious political actors rather than troublemakers.
Complete word intelligence in one call. Free tier — 50 lookups/day.