The lowest officer rank one can hold in the Army, Marine Corps, or Air Force; equivalent to the Navy's "ensign". All four are generically called an "O-1" by the DoD.
The three services that use the Army-style ranks abbreviate the rank as "2LT" on forms, which leads to the enlisted persons' term of endearment "L.T." Since they are otherwise required to call officers by their rank or by sir or ma'am, this term usually denotes the maximum acceptable amount of familiarity. Depending who you ask, referring to a second lieutenant as "L.T." may be considered more polite than "sir," especially if the officer and the enlisted person are co-workers.
In the world of military humor, 2LT's are like blondes. A second lieutenant is typically fresh out of college, and is placed in charge of as many as 60 enlisted personnel, 90% of whom have more time in uniform than the unfortunate L.T.; he is the authority, but doesn't actually have any experience or job-specific knowledge. If he's smart, he'll ask advice of his first sergeant; if not, hijinks ensue.