1. An unusually exact or detailed memory, often correlated with certain mental illnesses: When Janet had her 50th birthday, she discovered that she could recall and recite the poem she had memorised in elementary school, and her friends thought she had a case of hypermnesia!
2. An abnormally vivid remembrance of impressions apparently long forgotten: Some people experience hypermnesia when in an extreme situation, as in a moment of great danger when drowning.
3. A capacity under hypnosis for immediate registration and a precise recall of many more individual things than is thought possible under ordinary circumstances: The study of patients in a condition of sleep, which had been produced by suggestion, showed that their memories were extremely vivid and complete, and therefore supporting the fact of hypermnesia.
4. An extreme retentiveness or unusual clarity of memory: Although Tom's mother was very old and severely sick, she evidently had hypermnesia and remembered the exact composition and date of the recital where her piano teacher played when she was in college.