Abstract: A task to be achieved by the present invention is to provide an ink which can form a printed image or the like having color fastness to water at a level where the image will not be removed from the recording medium even when, for example, washed with water, warm water, or the like. Further, another task to be achieved by the invention is to provide an ink which can form a printed image or the like which is unlikely to suffer from color fading even when, for example, washed by an ultrasonic cleaning method or the like that is used for cleaning a stain strongly deposited on clothes or the like. The present invention is directed to an ink containing a polymer (A) having a predetermined structure (a1).