"Jedi" is a concept from the fictional Star Wars universe created by George Lucas. The Jedi (pronounced jedd-eye) are an order of quasi-religious warrior/philosophers modeled loosely on Buddhist monks and the Samurai. The Jedi have the gift of "Force sensitivity." The Force, in this fictional universe is an "energy field that connects all living things, it surrounds us, it penetrates us, it binds the galaxy together" (to paraphrase one Jedi named Obi-Wan Kenobi). The dark side of the Force is a form of evil, practiced primarily by fallen Jedi called the "Sith." The singular is "Jedi" and the plural is "Jedi."
The word "Jedi" was borrowed by George Lucas from a Japanese word that refers to historical films/period piece films in Japan set usually during the Samurai era.