Wikipedia
or "Single-minded school". It is usually viewed as a small, militant, offshoot from Jōdo Shinshū Buddhism though the name has a complex history.
Originally Ikkō-shū was an "obscure band of Pure Land proponents" founded by Ikkō Shunjō in the fifteenth century. He was a disciple of Ryōchū of the Chinzei branch of Jōdo-shū Buddhism) and similar to Ippen's Ji-shū. However, when the religious and military-political establishment began to crack down on the Nembutsu, little distinction was made between the various factions. Most of Ikkō Shunjo's followers therefore defected to the more powerful Jōdo Shinshū and the name Ikkō-shū ultimately became synonymous with Jōdo Shinshū.
Rennyo, the charismatic leader of the Hongan-ji branch of Jōdo Shinshū responded to this situation by clarifying the positive religious meaning of 'Ikkō' (single-minded) whilst simultaneously distancing himself from the antinomian behaviour of the original Ikkō sect. In his pastoral letters, known as Ofumi or Gobunsho, he therefore wrote; "It has been established with certainty that our Founder did not particularly name our school the "Ikkō-shū". On the whole, the reason the people call us this is that we place our complete reliance, exclusively, on Amida Buddha ...'However, the Founder has specifically named this sect "Jōdo Shinshū". Therefore, you must understand that we of our sect did not originate in any manner or form the name of "One-Mind Sect."