Iaido (居合道) (or Iaijutsu) is the art of drawing the katana, chopping down the opponent, flipping blood from the blade, and then re-sheathing the katana in one fluid movement. The emphasis is on drawing the sword and attacking as quickly as possible. Starting positions can be from combative postures or from everyday sitting or standing positions. Practitioners could expect a surprise attack at any time, and the ability to react quickly from an everyday starting position was considered essential.
An original date of 1200AD has been claimed, but some time during the late 15th or early 16th century is more likely. Most modern schools consider samurai called Hayashizaki Jinsuke Minamoto no Shigenobu (1546 - 1621?) as the originator of iaido. Not much is known about his life, and some scholars doubt his existence as a historical figure. The two largest schools of iaido that are practised today, Muso Shinden Ryu and Muso Jikiden Eishin Ryu, both claim a lineage starting with Hayashizaki.
While not a hard and fast rule, frequently the word iaido is used to refer to the modern self improvement oriented form taught by the Zen Nihon Kendo Renmei (ZNKR) and other iaido associations while iaijutsu is used for the older, combative techniques of the koryu. The word iaido itself was coined by Nakayama Hakudo in early 20th century. Before that various other names like batto, battojutsu, or saya no uchi were used instead.