 The nominal phrase N in a definite description “the N” contributes a condition that should be sufficient to discriminate the referent of the description from its local distractors. If I felicitously utter “the red one”, then you should be able to successfully identify the referent I have in mind on the basis of its being red. In this case, the condition that serves for identification in context is not specific enough to determine the referent outright—there are of course many red things in existence other than the one I refer to.