Starfleet borrows heavily from mainly the US navy but enough elements from other navies and changes to further suggest a "United Earth" service with less focus on pure military
I dont see the difference I've never been in the navy but I would wager that they use the term "first officer" as well since they're the same thing
"First officer" seems to be terminology in use among Starfleet command crews to describe the position of the second in command, primarily aboard starships. Scotty refers to Decker as "exec" in Star Trek: The Motion Picture, probably because the term "first officer" is one of both respect and acceptance in Starfleet, and Scotty did not know Decker that well, or trust him as he did Kirk, and, to him, Spock would always be "first officer".
In Star Trek: The Next Generation "Where No One Has Gone Before", Kosinski refers to Riker in addressed dialogue as "first officer", perhaps mockingly, or perhaps recognizing the uniqueness of its use not only among Starfleet command crews, but aboard the Enterprise-D ProfessorTrek 04:38, August 11, 2011 (UTC)
Then, of course, there is the term "Number One"...
While both terms mean the same thing, "First Officer" is often used in commonwealth navies interchangably with the American term "Executive Officer"Cpthunt 05:27, August 11, 2011 (UTC)