Wiktionary
n. A nervous condition which prevents a darts player from releasing the dart at the optimum moment.
Wikipedia
Dartitis (pronounced dart-eye-tis) is a condition which can affect darts players, and severely damage their performance. It can be compared to 'the yips', an expression used to describe apparent loss of fine motor skills without any explanation. The term is used in reference to players who struggle with some kind of psychological problem with their technique and/or release of their darts.
The most famous case of dartitis was in 1986, when five-times World Champion, Eric Bristow, revealed he was having problems with the release of his darts. Bristow managed to make a partial recovery from the condition, and even managed to regain the number one position in the world rankings.