also: もっぶ
noun
Refers to a large, often disorderly group of people. Can carry a negative nuance of an unruly crowd.
駅前にモブが集まって騒いでいた。
A mob gathered in front of the station and was making noise.
noun
background character; mob (in a game); NPC
Only with reading モブ
In manga, anime, and video games, refers to unnamed background characters or generic enemies (mobs). The reading モッブ is not used for this sense.
このゲームではモブを倒して経験値を稼ぐ。
In this game, you defeat mobs to earn experience points.
アニメの背景にいるモブキャラが意外と人気だ。
The background mob characters in the anime are surprisingly popular.
Borrowed from English 'mob'. The variant モッブ reflects an attempt to represent the English short 'o' sound, but モブ is the standard adaptation.