A decade is a division of troops serving on a ship in the Radch military. A decade contains at least ten troops, although it may also contain twenty or more.
On a Justice, there are ten decades, named after Amaat, the god the ship is named after, and the eight halves of the Four Emanations. Each unit in a decade contains twenty troops, although each decade contains multiple units - presumably ten, as the captain of a Justice is known as a Hundred Captain. Decades of lower numbers have seniority, so One Amaat outranks Two Amaat.
On a Mercy, there are a smaller number of decades. Each decade contains only a single unit of ten troops, except for the captain's decade (named for the ship's god) which contains twenty.
Every unit of troops is commanded by a Lieutenant, and on ships with multiple units per decade each Lieutenant reports to a Decade Commander. All decades on a ship report to the ship's Captain.