A large crowd. Often described as torch-wielding, a mob is a large (usually poorly educated or rough) crowd of perhaps villagers with pitchforks who move against certain things. Like a crowd of lemmings they follow one another.
Also used to describe the general populace.
GRACCHUS: I think he knows what Rome is. Rome is the mob. He will conjure magic for them and they will be distracted. He will take away their freedom, and still they will roar. The beating heart of Rome is not the marble of the Senate, it is the sand of the Colosseum. He will bring them death, and they will love him for it. --from Gladiator movie (2000)