"Social withdrawal" refers to the act of purposely spending less time with others or avoiding social interactions. It means preferring to be alone or isolating oneself from others. Full definition
Peer ratings of social withdrawal were collected within schools in 8th grade. (link.springer.com)
Social withdrawal in childhood and adolescence: Peer relationships and social competence. (psychologytoday.com)
In contrast, others mainly have «negative» symptoms, such as social withdrawal and diminished emotional and verbal expression. (scientificamerican.com)