Amnesia: systematic and extensive loss of “memory” caused by organic or psychological factors.The loss may be temporary or permanent, and may involve old or recent memories. (NCIt) A condition in which memory is impaired or lost. (OxfordMed) Abnormal forgetting caused by various forms of brain disease. (Goldberg, 109) 

Unusual forgetfulness. One may not be able to remember new events, recall one or more memories of the past, or both. Normal aging may cause some forgetfulness. It's normal to have some trouble learning new material, or needing more time to remember it. However, normal aging does not lead to dramatic memory loss. Such memory loss is due to other diseases. Sometimes, memory loss may be seen with “depression.” It can be hard to tell the difference between memory loss and confusion due to depression. (PubMedHealth2) Also referred to as ‘memory loss.’

Anterograde Amnesia: the inability to acquire new memories. (OxfordMed) Loss of ability to learn new information (Goldberg, 117) A condition in which people can remember the distant past and learn new skills, but they cannot form new memories for facts and events. (Chudler, 55) The most famous case of anterograde amnesia is that Henry Gustav Molaison (1926-2008), referred to as ‘HM.’ (Hockenbury, 248) Also referred to as ‘fugue amnesia.’

Retrograde Amnesia: loss of preexisting memories. (OxfordMed) Loss of ability to recall information acquired before the damage took place. (Goldberg, 118) Loss of the ability to recall information that had been previously encoded in memory prior to a specified or approximate point in time. May be associated with trauma, accidents, “seizures,”  “dementia,” and a wide variety of other conditions that impair cerebral function. (NCIt) People who have retrograde amnesia are unable to remember some or all of their past. (Hockenbury, 248)

Temporal Gradient of Retrograde Amnesia: the concept that when memory suffers, relatively recent memories will be more affected than memories for a very distant past. (Goldberg, 11)