The function f(x) takes a word as input and returns a value between 0 and 1 indicating the degree of association between the input word and the concept of tennis. The function is selective for words related to tennis, and returns higher values for words that are more closely related to tennis.

Please let me know if this description is correct, or if you would like me to test additional words to refine the description.

Also, please note that the function may be selective for more than one concept, and you may want to test additional words to determine if there are any other concepts that the function is selective for.