a commissioned military officer
"The lieutenant ordered his squad to secure the perimeter before dawn."
an officer in a police force
"The lieutenant arrived at the scene to direct traffic after the accident."
an assistant with power to act when his superior is absent
"When the captain was stuck in traffic, the lieutenant took command and ordered everyone to evacuate immediately."
an officer holding a commissioned rank in the United States Navy or the United States Coast Guard; below lieutenant commander and above lieutenant junior grade
"After graduating from naval academy, she was promoted to lieutenant before receiving her next commission as a lieutenant commander."
The lowest Junior Commissioned Officer rank(s) in many military forces, often Army and Marines.
"The captain nodded in acknowledgment to Lieutenant Smith as he approached the bridge."
In the US Army, Air Force and Marines, holding the rank above Second Lieutenant and below Captain. Both ranks may be referred to as "Lieutenant" or as the complete forms of the ranks.
Alternative letter-case form of lieutenant (when used as a title before a name)
A military grade that is junior to the grade the adjective modifies: lieutenant colonel, lieutenant general, lieutenant commander.
"After years of service as a captain, he finally advanced to the rank of lieutenant colonel and took command of the battalion."