retentiveness C2+

/rɪˈtɛntɪvnəs/ the capacity to retain or remember things, especially knowledge

retainer C2+

/rɪˈteɪnɚ/ a fee paid to secure services; a servant or device that holds something in place

retentivity C2+

/ˌritɛnˈtɪvəti/ the capacity to retain something, especially memory or magnetism