There are physical entities who have only one psyche or consciousness within their body. This the most common societal depiction of a person. (Compare: any representation media-wise of a singular vs group entity.)
The term "singleton" (which already meant a single person or thing of a subject under consideration i.e. a singleton within a family unit) was adopted from general use to differentiate between multiples and non-multiples, and has fallen out of use due to the similarity to "simpleton" — the words are not related other than etymology (the words are constructed similarly).
Now "singlet" and "singular" are being used to indicate individuals who are non-plural, since it's wrong to imply that plurality or multiplicity in itself is "wrong" or "abnormal" — or that all singular people are free of their own issues — by saying that singular entities are inherently "normal people." Implying that singularity in itself grants a person "normality" is a potentially offensive concept to members of both groups.
Also compare the strict definition of "one mind" or "one person" to concepts in psychology that include that everyone has facets, roles or aspects that are, at the very least "personality states" or "self-states". This concept is being explored in many areas of the manual, such as:
- Born Multiple
- Everyday multiplicity
- Everyday evidence
- State-dependent memory
- Environment-dependent states
Alternately one can interpret a singular entity (using everyday multiplicity, self-pluralism, and other models), as someone who falls within the "average" spectrum of multiplicity. Defining "average", however, might be a daunting task. ;)