A short sword is a one-handed melee weapon. They are slightly weaker than long swords. Because of their inefficient bonuses, short swords are not popular among RuneScape Classic players, although they are sometimes used by inexperienced players.
Short swords up to adamantite can be purchased from Varrock Swords, and the Rune Short Sword can be purchased from Scavvo's Rune Shop in the Champions' Guild. All of them can also be made using the Smithing skill, and require 1 bar of the relevant metal.