Sensory processing sensitivity (SPS) is a temperamental or personality trait involving "an increased sensitivity of the central nervous system and a deeper cognitive processing of physical, social, and emotional stimuli". The trait is characterized by "a tendency to 'pause to check' in novel situations, greater sensitivity to subtle stimuli, and the engagement of deeper cognitive processing strategies for employing coping actions, all of which is driven by heightened emotional reactivity, both positive and negative".

A human with a particularly high measure of SPS is considered to have "hypersensitivity", or be a highly sensitive person (HSP). The terms SPS and HSP were coined in the mid-1990s by psychologists Elaine Aron and her husband Arthur Aron, who developed the Highly Sensitive Person Scale (HSPS) questionnaire by which SPS is measured. Other researchers have applied various other terms to denote this responsiveness to stimuli that is seen in humans and other species.

According to the Arons and colleagues, people with high SPS make up about 15–20% of the population. Although some researchers consistently related high SPS to negative outcomes, other researchers have associated it with increased responsiveness to both positive and negative influences. Aron and colleagues state that the high-SPS personality trait is not a disorder.

Elaine Aron's book The Highly Sensitive Person was published in 1996. In 1997 Elaine and Arthur Aron formally identified sensory processing sensitivity (SPS) as the defining trait of highly sensitive persons (HSPs). The popular terms hypersensitivity (not to be confused with the medical physiological term hypersensitivity) or highly sensitive are popular synonyms for the scientific concept of SPS. By way of definition, Aron and Aron (1997) wrote that sensory processing here refers not to the sense organs themselves, but to what occurs as sensory information is transmitted to or processed in the brain. They assert that the trait is not a disorder but an innate survival strategy that has both advantages and disadvantages.

Elaine Aron's academic journal articles as well as self-help publications for the lay reader have focused on distinguishing high SPS from socially reticent behavior and disorders with which high SPS can be confused; overcoming the social unacceptability that can cause low self-esteem; and emphasizing the advantages of high SPS to balance the disadvantages emphasized by others.

In 2015, journalist Elizabeth Bernstein wrote in The Wall Street Journal that HSPs were "having a moment," noting that several hundred research studies had been conducted on topics related to HSPs' high sensitivity. The First International Scientific Conference on High Sensitivity or Sensory Processing Sensitivity was held at the Vrije Universiteit Brussel. By 2015, more than a million copies of The Highly Sensitive Person had been sold.

Research pre-dating the Arons' coining of the term "high sensitivity" includes that of German medicine professor Wolfgang Klages, who argued in the 1970s that the phenomenon of sensitive and highly sensitive humans is "biologically anchored" and that the "stimulus threshold of the thalamus" is much lower in these persons. As a result, said Klages, there is a higher permeability for incoming signals from afferent nerve fibers so that they pass "unfiltered" to the cerebral cortex.

The Arons (1997) recognized psychologist Albert Mehrabian's (1976, 1980, 1991) concept of filtering the "irrelevant", but wrote that the concept implied that the inability of HSPs' (Mehrabian's "low screeners") to filter out what is irrelevant would imply that what is relevant is determined from the perspective of non-HSPs ("high screeners").