Retentiveness is the name for how well a mind holds on to what it has learned. If your retentiveness is strong, facts stay with you. It is a heavy, formal noun. Students and teachers almost always say “memory” or “a good memory” instead. You will meet it in older books or in careful writing more than in chat. Though actually, people mix it with retention, which is a wider word: retention can mean keeping water, keeping a job, or a school keeping its students — not only a mind keeping facts.