A Second Officer was third in command of a vessel or ship, and normally a department head as well.
Some starships, typically those on deep space missions, kept the position separate from other duties.
Second Officers[]
Montgomery Scott - USS Enterprise and USS Enterprise-A
Gedna Tachion - USS Ambassador (2351 - 2357)
Data - USS Enterprise-D and USS Enterprise-E
Jadzia Dax - Deep Space Nine (2369 - 2374) and USS Defiant (2372 - 2374)
Nikolas Stone - USS Hargrave (2368 - 2371) and USS Lionheart (2371 - 2372)
Seff O'Rourke - USS Lionheart (2373)
Jobe - USS Lionheart (2373)
Rebecca Gallagher - USS Camelot (NCC-73220) (2374)