A person of high social status in Japan. One who had access and control over both samurai and shinobi, sometimes in vast numbers. If a daimyo gave an order to one of his samurai or shinobiwarriors, then that warrior must complete that order or commit sepokku (ritualistic suicide performed when one is dishonored)
Did you see that guy? he obeyed every word his girlfriend said! He was even bowing to her! Is she his daimyo or something??"
Each samurai had a lord in which he served. The Daimyo were known for their skill as generals and military leaders. Following the shogun, the daimyo were the most powerful rulers from the 10th century to the early 19th. The daimyo were only subordinate to the Shogun and only one man at a time could be Shogun. There were many daimyos all across Japan, but there was only one Shogun at a time. The samurai who had no lord were known as Ronin.
A warlord and a high ranking individual in Medieval and Tokugawa Japan, a person of power and great wealth, were kept powerless when in Edo, by of Tokugawa Ieyasu's will, and were an "ally" to the Shogunate's advantage, there were 260 daimyo in all of Japan in the Tokugawa period, and were kept in their place, and were all kept pleased by the Shogun, or rebellion would be a major factor in Japan. Owned vast armies of samurai, and lands, and a castle estate.
Daimyo were like a version of a nobleman or a king in today's standards within their own domains and by their people they ruled over.