Transience. A weakening or loss of memory over time. It's probably not difficult for you to remember now what you have been doing for the past several hours. But if I ask you about the same activities six weeks, six months, or six years from now, chances are you'll remember less and less. Transience is a basic feature of memory, and the culprit in many memory problems.

Absent-mindedness. A breakdown at the interface between attention and memory. Absent-minded memory errors — misplacing keys or eyeglasses, or forgetting a lunch appointment — typically occur because we are preoccupied with distracting issues or concerns, and don't focus attention on what we need to remember. The desired information isn't lost over time; it is either never registered in memory to begin with, or not sought after at the moment it is needed, because attention is focused elsewhere.

Blocking. Somewhere between remembering and forgetting lies blocking. You know that the word for an oration at a funeral begins with a vowel, maybe even a “u”... but it just won't spring into consciousness.

Misattribution. The El Al cargo flight had smashed into an apartment building outside Amsterdam, killing 39 residents and all four crew members in a fiery explosion. Ten months after the 1992 disaster, Dutch psychologists quizzed colleagues about how well they remembered television footage of the crash. Most remembered it so well that they could describe whether the fuselage was aflame before it hit, where the plane fell after impact and other details. But there was no such footage: people attributed to video what they had inferred from newspapers, discussions with friends and other sources.

Suggestibility. People confuse personal recollection with outside sources of information. This has profound implications in legal settings.

Bias. It is a cliche that couples in love recall their courtship as a time of bliss, while unhappy pairs recall that "I never really loved him [or her]." But the cliche is true. We rewrite our memories of the past to fit our present views and needs. Stereotyping can also bias memory. Researchers at Yale University asked students which names on a list they recalled as those of criminals recently in the news. The students were twice as likely to "remember" the stereotypically black names ("Tyrone Washington") as they were the stereotypically white ones ("Adam McCarthy"). None of the names had been in the news as criminal suspects or anything else, the scientists reported in 1999. When memory conflicts with what you're convinced is true, it often comes out on the losing end.

Persistence. Memories that refuse to fade tend to involve regret, trauma and other potent negative emotions. All emotions strengthen a memory, but negative ones seem to write on the brain in indelible ink.