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a noisy, violent public disorder caused by a group or crowd of persons, as by a crowd protesting against another group, a government policy, etc., in the streets.
Law. a disturbance of the public peace by three or more persons acting together in a disrupting and tumultuous manner in carrying out their private purposes.
violent or wild disorder or confusion.
a brilliant display:
a riot of color.
something or someone hilariously funny:
You were a riot at the party.
unrestrained revelry.
an unbridled outbreak, as of emotions, passions, etc.
Archaic. loose, wanton living; profligacy.
to take part in a riot or disorderly public outbreak.
to live in a loose or wanton manner; indulge in unrestrained revelry:
Many of the Roman emperors rioted notoriously.
Hunting. (of a hound or pack) to pursue an animal other than the intended quarry.
to indulge unrestrainedly; run riot.
to spend (money, time, etc.) in living (usually followed by away or out).
run riot,

to act without control or restraint:
The neighbors let their children run riot.
to grow luxuriantly or abundantly:
Crab grass is running riot in our lawn.

Contemporary Examples

The scene is a riot of excess, soaring voice decibels—and precious.
Eight Years After Hurricane Katrina, New Orleans Has Been Resurrected Jason Berry August 28, 2013

The mob flipped over a news van, tore down two lampposts, and threw rocks and cans; police responded with riot gear and tear gas.
Penn State Loses First Paterno-Less Game November 11, 2011

Surging crowds of demonstrators faced off with riot police along a heavily guarded street.
Tahrir Square’s Original Activists Wonder If Arab Spring Over as Protests Change Mike Giglio September 13, 2012

Millions are expected to turn out across the country, and clashes between demonstrators and riot police seem inevitable.
Egypt Braces For a Fight Mike Giglio June 27, 2013

They made one last charge for the airport, and when the riot police blocked them again a melee ensued.
Mexican Protesters Look to Start a New Revolution Jason McGahan November 20, 2014

Historical Examples

riot and disorder too often characterized the events of that season.
A Clerk of Oxford Evelyn Everett-Green

The riot had now changed into open mutiny among these marshmen and miners.
Micah Clarke Arthur Conan Doyle

I had cast off all feeling, subdued all anguish to riot in the excess of my despair.
Frankenstein Mary Shelley

There was a riot in the streets; in one house the poison cup was ready.
Things as They Are Amy Wilson-Carmichael

Then ensued a scene of riot and carnage such as no human pen, or steel one either, could describe.
Sketches New and Old, Complete Mark Twain (Samuel Clemens)

noun

a disturbance made by an unruly mob or (in law) three or more persons; tumult or uproar
(as modifier): a riot gun, riot police, a riot shield

boisterous activity; unrestrained revelry
an occasion of boisterous merriment
(slang) a person who occasions boisterous merriment
a dazzling or arresting display: a riot of colour
(hunting) the indiscriminate following of any scent by hounds
(archaic) wanton lasciviousness
run riot

to behave wildly and without restraint
(of plants) to grow rankly or profusely

verb
(intransitive) to take part in a riot
(intransitive) to indulge in unrestrained revelry or merriment
(transitive) foll by away. to spend (time or money) in wanton or loose living: he has rioted away his life
n.

c.1200, “debauchery, extravagance, wanton living,” from Old French riote (12c.) “dispute, quarrel, (tedious) talk, chattering, argument, domestic strife,” also a euphemism for “sexual intercourse,” of uncertain origin. Cf. Medieval Latin riota “quarrel, dispute, uproar, riot.” Perhaps from Latin rugire “to roar.” Meaning “public disturbance” is first recorded late 14c. Meaning “something spectacularly successful” first recorded 1909 in theater slang.

Run riot is first recorded 1520s, a metaphoric extension from Middle English meaning in reference to hounds following the wrong scent. The Riot Act, part of which must be read to a mob before active measures can be taken, was passed 1714 (1 Geo. I, st.2, c.5). Riot girl and alternative form riot grrl first recorded 1992.
v.

late 14c., “behave in a dissolute manner, engage in loose revelry,” from Old French rioter “chatter, dispute, quarrel,” from riote (see riot (n.)). Meaning “take part in a public disturbance” is from 1755. Related: Rioted; rioting.
see:

read the riot act
run amok (riot)