An executioner's sword is a sword whose blade is almost completely rectangular--the end does not curve to a tip or form a point, but simply ends rather suddenly, as shown here. Such blades were usually used for ceremonial purposes or executions (hence the name).
Swords like these are typically western in design, and should not be confused with weapons like the khanda, a sword from India that was actually meant to be used in battle.
Despite the historically ceremonial/special-use nature of an executioner's sword, this has not stopped them from being occasionally portrayed as legitimate weapons in media and fanart.
