The short form of the Loneliness Scale measures subjective social isolation (section: mental health), which is defined as a situation experienced by the participant as one where there is an unpleasant or inadmissible lack of (quality of) certain relationships. It includes situations where the number of existing relationships is smaller than desirable or acceptable, as well as situations where the intimacy wished for has not been realized¹⁾.
A manual to analyze the scale can be found here. The scale is used widely in the Netherlands (the original scale is Dutch) but also internationally and has shown good psychometric properties for overall, emotional, and social loneliness that is suitable for large surveys²⁾.