Nindō (忍道, Literally meaning: Ninja Way) is a personal rule that each shinobi lives by. It is their way of ninja life, their motto, belief, or "dream". A ninja's nindō can be anything, no matter how immoral their personal point of view is. Most often Naruto Uzumaki proclaims his ninja way, to be: "I'm not gonna run away and I never go back on my word, that is my nindō! My ninja way!" There is nothing stopping other ninja from following another ninja's personal rule, such as Hinata Hyūga and Jiraiya adopting Naruto's nindō, or Rock Lee following Might Guy's "My Rules" philosophy. There is also nothing wrong with a ninja following more than one nindō, such as the above Konoha ninja also following the Will of Fire belief from their village in addition to their own personal rules.
According to Tobi, the nindō of the Uchiha clan is to exact revenge against their hated enemies. Conversely, the Senju created what is known as the Will of Fire in Konohagakure, which can be said to be the universal ninja way of all the shinobi of this village. In Iwagakure exists a similar philosophy known as the Will of Stone, which simply states that shinobi of the village must be strong willed. Kisame Hoshigaki's philosophy as taught to him by Fuguki Suikazan is that confidential information is more important than human lives, and he would go as far as to betray comrades, and even commit suicide to ensure that it remains secret.