A bastard sword, also known as a longsword, is a type of Europeanheavysword which is so named because it can either be wielded with one or both hands.
The bastard sword is a double-edged, straight sword of European origin which has been dramatically up-scaled, much like its cousins, the Scottishclaymore and the GermanZweihänder. While the claymore is characterized by its distinct cross-guard and the Zweihänder typically incorporates a ricasso, the bastard sword tends to be a more general category reserved for greatswords which fall into neither of these two later designs. Its defining characteristic, however, is that it remains just light and short enough for it to be physically wielded with one hand by strong swordsmen. Nonetheless, because a sword of this size is still optimally wielded with two hands, bastard swords sit at the border between "two families" of swords, resulting in their bastard moniker.
In real-life, bastard swords are also known as longswords or hand-and-half swords. In the Assassin's Creed series, however, two-handed swords are haphazardly identified as bastard swords, longswords, or claymores without distinction, resulting in many errors.