These are a type of Capital Ship designed to partake in space warfare. They carry less armour than battleships in order to gain faster speeds. They occupy a position between cruisers and battleships. They're typically the largest and strongest warships that aren't battleships.
Battlecruisers are long range, independent ships that can control entire systems. They're heavily armed and capable of deploying large fighter squadrons. Battlecruisers can also carry out ambassadorial missions with ‘soft’ power projection, as well as hard.