Introverted Sensation (or "Sensing") is one of the eight function-attitudes defined by Carl Jung.
Sensation (S) is an outlook that focuses on tangible, material or practical data and/or experience, which is generally “itemized” (i.e. attention to "details"), where existence is thought of in terms of what simply “is” or “isn’t”, or "the substance of" things.
An "introverted" attitude (i) indicates that this outlook generally draws from the individual; or the inner world of the "subject" (individual ego), instead of directly from the outer world of the environment.
So introverted Sensing (Si) can be described as "awareness of material reality filtered by individual knowledge", where one turns inward to compare at hand data such as physical/practical items with a storehouse of fact and experience. This "internal" perspective will also include internal bodily sensations (hunger, pain, etc.) Every type does these things, but for types who prefer Si, it will be their main form of information-gathering, rather than sensory "facts" directly from the environment (Se), or intangible data such as inferences or hypothesis (N). These types will be denoted in the type code by an "S and "J"; i.e. "xSxJ"; the "J" pointing to the preferred "Judgment" function, Thinking or Feeling, as being what is extraverted, and so for cognitive balance, Sensing, as the perception function will be introverted or attuned to the individual.
An introverted Sensing preference will be marked by attention to remembered detail, which current data is filtered through. It is often associated with "memory", which it does rely a lot on, but it is not accurate to say that the function is equal to memory by itself. The functions are cognitive interpretations of data and Si is an interpretation of the data in light of what is remembered. Types preferring Si often desire familiarity.