a situation in which a large crowd of people are behaving in a violent and uncontrolled way, especially when they are protesting about something:
His murder triggered vicious race riots (=caused by a problem between different races).
police wearing riot gear (=the special clothing and equipment worn by police officers during a riot)
police in bullet-proof vests and carrying riot shields (=a piece of very hard plastic which police officers stand behind to protect them)
if your imagination, emotions, thoughts etc run riot, you cannot or do not control them:
Manufacturers have let their imaginations run riot to create new computer games.
if people run riot, they behave in a violent, noisy, and uncontrolled way:
Some people let their children run riot.
if a plant runs riot, it grows very quickly
something with many different bright colours:
The garden is a riot of colour in spring.
to give someone a strong warning that they must stop causing trouble - used humorously:
If the kids don't settle down soon, I'll go up and read them the riot act.