Dots per inch. You have to know the dots, but you also have to know the inches. What are the dimensions if you turn the rulers on in Photoshop?  
For instance for a 5 x 7 card, an image that is 240 dpi that is only 3 inches square would not be high enough resolution.  If the image were 20 inches tall, however, it could be resized down to 5 x 7 and 300 dpi, without loss of quality.
Therefore, you need to know both the dpi AND the measurements to determine the answer to your question.