Lenore Thomson defines it as follows:
"Introverted Perception dictates an interest in represented experience--words, facts, numbers, signs, and symbols: the kind of data that can be acquired or explored in the mind."
Here's a further definition of introverted perception from the Lenore Thomson wiki website:
A proposed definition: categorization without criteria.
Perhaps what Lenore means by Introverted Perception is an attitude of putting things you experience into categories, where the categories are chosen on the basis of whatever seems to you to fit the things, without serving a predefined purpose or criterion.
Taking an Si perspective, then, you simply find categories to put things in, and use these categories to build a rich network of mental associations that guide you to attend to the things that matter to you--to find those things in the midst of a predominantly overwhelming perceptual field.
Taking an Ni perspective, you attend to the nature of whatever categories you come across: what they contain and what they leave out, what they assume about the context where they're applied, what (probably unstated) purposes are served by those categories, what cannot be said in terms of those categories (and that, if said, might unravel their power to seem real and meaningful).
Si leads you to gradually accumulate a factual map of the world, or at least the parts of it that are of interest to you. Ni leads you to gradually accumulate an understanding of how different maps operate and to be able to compare the assumptions of different maps against each other.
In my experience (as a dominant Introverted Perceiver) this is how Si and Ni seem to operate in all 16 types. In Si and Ni dominant types (IJs) the need to refer to an internal map (Si) or system of maps (Ni) is of paramount importance to the way we understand reality.
This can make us seem very strange or eccentric to an outside observer, as our preferred response to an external stimulus, event or question is to "go inside" to relate it to our internal perceptual structure.
If we are prevented from doing this, say, by a rapid-fire series of questions, or a situation demanding an immediate response without time for internal pondering, then we may become very disorientated and confused. This is the exact opposite of extraverted perceiving types (EPs), who thrive on rapidly changing or stimulating external events, but get very lost, confused or even depressed when forced to explore their internal perceptions.