Qu'est-ce qu'un third? Qu'est-ce qu'un third? The ordinal number matching the number three in a series.One of three equal parts.Music An interval of three degrees in a diatonic scale. Music A tone separated by three degrees from a given tone, especially the third tone of a scale.The transmission gear or gear ratio used to produce forward speeds next higher to those of second in a motor vehicle.Baseball Third base.Merchandise whose quality is below the standard set for seconds.