A derogatory way to refer to a soldier or member of the military (the term is common across South America and often implies the person has a low level of education or few career prospects).
After he couldn't get into any university, he went and became a milico.
A member of the police or military forces. (In Peru, this term is frequently used to refer to an individual officer rather than just the collective force.)
I ran the red light and a police efectivo pulled me over.
Relating to war. In Mexican slang, it describes someone who acts tough, aggressive, or displays the lifestyle and aesthetic of organized crime (the term is currently popularized by regional music subcultures).