Real Dictionary

mob
countable noun: A mob is a large, disorganized, and often violent crowd of people.
singular noun: People sometimes use the mob to refer in a disapproving way to the majority of people in a country or place, especially when these people are behaving in a violent or uncontrolled way.
singular noun: You can refer to the people involved in organized crime as the Mob.

mob in American English
a disorderly and lawless crowd; rabble
adjective: of, pertaining to, or characteristic of a lawless, irrational, disorderly, or riotous crowd
noun: a disorderly or riotous crowd of people
transitive verb: to crowd around noisily, as from curiosity or hostility

mob in British English
abbreviation for: mobile phone
noun: a riotous or disorderly crowd of people; rabble
verb: to attack in a group resembling a mob