One who demonstrates; a person who shows, explains, or proves something, or a person who participates in a public demonstration.
From demonstrate + -er (a suffix forming agent nouns indicating one who does the action). Essentially a variant of demonstrator with slightly different spelling.
'Demonstrater' and 'demonstrator' are both valid but 'demonstrator' won out because it sounds more authoritative—good example of how certain word endings feel 'right' in English even when alternatives work perfectly.
Complete word intelligence in one call. Free tier — 50 lookups/day.