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wikipedia
In 1992, the Indianapolis Motor Speedway trademarked the IndyCar name and licenced it to CART, which in turn renamed its championship the IndyCar World Series.
During the 1996 season, the IndyCar name was the subject of a fierce legal battle. Prior to the 1996 season, Indianapolis Motor Speedway President Tony George had created his own national championship racing series called the Indy Racing League. In March of 1996, CART filed a lawsuit against the Indianapolis Motor Speedway in an effort to protect their licence of the IndyCar name which the Indianapolis Motor Speedway had attempted to terminate. In April, the Speedway filed a counter suit against CART to prevent them from further use of the name. Eventually a settlement was reached in which CART agreed to give up the use of the IndyCar name following the 1996 season and the IRL could not use the name before the end of the 2002 season. Following a six-year hiatus, the IRL announced it would rename its premier series the IRL IndyCar Series for the 2003 racing season.
Nowadays, the Indy Racing League is the sole owner of the name IndyCar, and the use of the term for any other racing series is incorrect.