A longsword is a one-handed sword, a characteristic weapon of Westerosi knights.
Size relative to other swords
Bastard swords are generally larger than longswords. A shortsword is a smaller type of one-handed sword. A typical longsword is estimated as about a metre long (3 feet and 4 inches), though the size of the sword depends on the wielder.
The longsword has a long blade for a one-handed weapon, and most longswords are well suited to both cutting and thrusting.
Confusion with medieval "longswords"
In the context of the Middle Ages, a longsword generally refers to a hand-and-a-half sword or a bastard sword. The terms are largely interchangeable. However, in a Westerosi context, the longsword is exclusively a one-handed weapon, although they may have hilts long enough to allow limited two-handed use.