Kodachi: literally translating into “small or short tachi (sword)”, is a Japanese sword that is too short to be considered a long sword but too long to be a dagger. Because of its size, it could be drawn and swung extremely quickly. Thus, it could be used as something of a shield, while using a form of hand to hand combat to attack. Since this sword was shorter than two shaku (about two feet) in length, it did not exceed the blade length limits of non-samurai during the Edo period and could be worn by merchants.
A kodachi’s length is similar to that of the wakizashi, and though the blades differ greatly in construction, the kodachi and the wakizashi are similar enough in size and technique that the terms are sometimes misused interchangeably. While the kodachi was a set length, the wakizashi was forged to complement the height of its wielder or the length of the katana it was paired with, and thus varied. The kodachi also features greater curvature than a wakizashi, and typically has a longer handle.
The prefix “ko-” means “short,” and can be attached to any of the names of specific types of swords to indicate something shorter than “normal.” As the prefix “O-” can mean “great” or “long,” it follows that the opposite end of this length spectrum of the tachi is the odachi.