You are looking for two Form API entries:
Values: String. Possible values: One or more jQuery events, with multiple events separated by space characters.
Description: Provides an array of sections which are parts of $form_state['values'] which should be validated, implying that sections which are not listed should not be validated. This is normally used in multistep forms in the case of a "back" button, for example, where '#limit_validation_errors' => array() would mean not to validate anything as form values on the current page are to be discarded anyway. #limit_validation_errors does not have any effect if #submit is not set. More discussion is in the form_set_error() documentation.
Values: array indicating sections of the $form_state['values'] which should be validated.