The Old Man
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
The Old Man is the nickname amongst Galactica's crew for the battlestar's commanding officer, William Adama.
Note
"Old Man" is a traditional naval term used as slang for the master of a British Merchant Navy ship. It is also used in other countries' navies as a term for the commanding officer of a ship, regardless of gender, as well as the commander of a unit in various armies.