IPA: /ˈraɪətər/
KK: /ˈraɪətər/
A person who participates in a violent public disturbance or protest, often involving a crowd acting in a disorderly manner.
The police arrested several rioters after the protest turned violent.
Rioter → It is formed from "riot" (from Old French *riote*, meaning a noisy quarrel or disturbance) and "-er" (meaning a person who). A rioter is a person who participates in a noisy disturbance or public disorder.
Think of someone involved in a 'noisy quarrel' ('riot') — the '-er' just means a person. A rioter is someone who causes or takes part in a disturbance.