A Battleship is a type of capital ship that's the biggest, heaviest warship in a navy. It's usually only armed for combat, but can also transport troops and large fighter squadrons. It can take command of entire systems and conduct peace-keeping missions.
Battleships are larger, better armed and armoured, but slower and less agile than battlecruisers. They can do orbital strikes of targeted areas on planets. A direct hit from an orbital bombardment is very destructive. Additionally, it can provide support for planetary invasions. They often serve as flagships.
There are currently no battleships in-game, but they have been mentioned in the lore and as a ship class to fulfill specific roles in a fleet.