Definitions
1.nounThe lowest Junior Commissioned Officer rank(s) in many military forces, often Army and Marines.
2.nounA person who manages or executes the plans and directives of another, more senior person - i.e. a manager to his director.
3.nounThe second-in-command (2IC) of a group.
4.adjectiveA military grade that is junior to the grade the adjective modifies: lieutenant colonel, lieutenant general, lieutenant commander.