Answer: xanthosis
Akeelah remembered Dylan telling her it started with an X, but nevertheless spelled it zanthosis, trying to give the competition to Dylan in order to please his father. Dylan, not wanting to win unless Akeelah tried her best, also threw the word, spelling it xanthoses. Afterwards, he asked the judges for some water. This was also the winning word from the [real] national spelling bee in 1995.