Tomahawk

The tomahawk was a type of one-handed axe, which was most effective when wielded in close combat. It was commonly used by Native Americans, as well as European colonists, before modern times.

The tomahawk was a useful weapon, as it could be easily used for stealth kills, melee combat, or to perform menial tasks, such as cutting a rope.

During the American Revolutionary War, the Mohawk Assassin Connor Kenway used a tomahawk with a head resembling the Assassin insignia, which he would often dual-wield alongside another weapon, such as the Hidden Blade.

It appears to be the replacement of the Sword which was present in the previous installments.

Source

 * GameInformer April 2012 issue: Assassin's Creed III