Back to basics in the field of loneliness: progressing conceptualisation and definition of the term - an umbrella concept analysis.

The absence of a comprehensive, unified, conceptualisation of loneliness and the consequent lack of a clear and precise theoretical definition of loneliness impede research, policy and practice activities to understand and address this global public health issue. Our study aimed to establish the first such conceptualisation and develop the first such definition. To do so, we undertook a systematic conceptual review, specifically an umbrella concept analysis, including 42 documents summarising/synthesising the literature concerning the conceptualisation and/or theoretical definition of loneliness. The novel definition developed is the negative feeling(s) one experiences as a result of a (conscious or subconscious) personal perception that one's interpersonal needs are not satisfied by (the quantity and/or quality of) one's interpersonal (emotional, social, collective, professional and/or religious) relationships. In the process, we identified the unidimensionality of loneliness and generated clarity regarding the opposite of loneliness ('unloneliness'). We call on researchers, policymakers and practitioners working in the field of loneliness, the wider field of interpersonal relationships or encountering loneliness in other fields of activity, across the globe, to employ the novel conceptualisation and theoretical definition as a foundation for activities to further progress understanding and addressing of loneliness. We also encourage consideration of unloneliness, when undertaking such activities.