minimi
The other two definitions are correct, but omit the origin of the term. It was originally "mini-mittreilleuse" (not sure of spelling on that), a French neologism meaning "small machinegun". It's a 5.56mm weapon that fires the same NATO standard rifle ammo as the M16. It is normally fed from 200-rd belts, but can also accept 30-rd M16 magazines.
Example:
The Minimi is not a popular weapon because it is prone to malfunctions and the 5.56mm projectiles don't penetrate much in an urban environment.
The Minimi is not a popular weapon because it is prone to malfunctions and the 5.56mm projectiles don't penetrate much in an urban environment.