Eidetic or photographic memory is popularly defined as the ability to recall images, sounds, or objects in memory with extreme precision and in abundant volume. The word eidetic (pronounced /aɪˈdɛtɨk/) means related to extraordinarily detailed and vivid recall of visual images, and comes from the Greek word εἶδος (eidos), which is directly translatable to "seen" (as a past participle of "see").
Eidetic memory as observed in children is typified by the ability of an individual to study an image for approximately 30 seconds, and maintain a nearly perfect photographic memory of that image for a short time once it has been removed—indeed such eidetickers claim to "see" the image on the blank canvas as vividly and in as perfect detail as if it were still there. Much like any other memory, the intensity of the recall may be subject to several factors such as duration and frequency of exposure to the stimulus, conscious observation, relevance to the person, etc. This fact stands in contrast to the general misinterpretation of the term which assumes a constant and total recall of all events.
Some people who generally have a good memory claim to have eidetic memory. However, there are distinct differences in the manner in which information is processed. People who have a generally capable memory often use mnemonic devices (such as division of an idea into enumerable elements) to retain information while those with eidetic memory remember very specific details, such as where a person was standing, what the person was wearing, etc. They may recall an event with greater detail while those with a different memory remember daily routines rather than specific details that may have interrupted a routine. However, this process is generally most evident when those with eidetic memory make an effort to remember such details.
Also, it is not uncommon that some people may experience 'sporadic eidetic memory', where they may describe some number of memories in very close detail. These sporadic occurrences of eidetic memory are not triggered consciously in most cases.
Cases of eidetic memory in Aliens have been observed. Trained Vulcans posses the ability and every Ane so far tested likewise has passed every test for eidetic memory. Science as a body of work has yet to render a verdict.
See: Wikipedia Eidetic Memory for more.