Originally, "kimono" was the Japanese word for clothing. However, in recent years, the word has been used to refer specifically to traditional Japanese clothing. Kimono as we know them today came into the form during the Heian period (794-1185).

From ancient times, and all the way through the Nara period (710-794), Japanese people typically wore either ensembles consisting of separate upper and lower garments (trousers or skirts), or one-piece garments.

During the Meiji period (1868-1912), Japan was heavily influenced by foreign cultures. The government encouraged people to adopt Western clothing and habits. Government officials and military personnel were required by law to wear Western clothing for official functions. (That law is no longer in effect today.) Ordinary citizens, on the other hand, were required to wear kimono decorated with their family crests, called kamon, which identified their family backgrounds, on formal occasions. This type of kimono is called mantsuki. Since then, montsuki became formal wear for Japanese people.

More Info: web-japan.org