Either a phenomenon or, more interestingly, a moment of Perception. As a perception it is closely analogous to the Sense of Wonder in Science Fiction, which may be defined as a shift in perspective so that the reader, having been made suddenly aware of the true scale of an event or venue, responds to the revelation with awe. The analogue in fantasy is the discovery by the reader that there is an immense gap between the time of the tale and the origin of whatever it is that has changed one's perspective on the world. The TA may be occasioned by almost anything: a Rune, a Name, a memory, a Land, an artefact, the Wandering Jew ... In Alan Garner's Elidor (1965), when the Childe Roland discovers that his siblings have been profoundly immured – indeed, have fossilized – in a kind of amber for thousands of years, though only a short subjective time has passed for him, what is then experienced is a jolt of perception. Similarly, in Lloyd Alexander's The Book of Three (1964), the discovery of the bones of a ship in a hidden valley becomes a TA when those bones turn out to be those of Noah's Ark. And J R R Tolkien's The Lord of the Rings (1954-1955) – once the immense backstory contained in The Silmarillion (1977) and other texts is understood – seems to hover at the very lip of a profound TA.
There is a central moment in many fantasy narratives when the protagonist recognizes the Story he has been living and remembers who he truly is. Depending on the nature of the story, and of the act of remembering, a TA may be experienced. Various protagonists in Robert Holdstock's Ryhope Wood sequence experience TAs of this sort, as they discover their own faces staring out at them from the heart of the woods and from the depths of time.
In fantasy, the TA almost always marks a gap between the present of the tale and some point deep in the past. Where an artefact or vista signals a gap between the tale's present and some point in the future, this is more likely to be perceived as an Anachronism. [JC]