Socialization is an important part of the healthy aging process, but natural changes in the lifestyle and health of older people increased risk of loneliness. However, loneliness is not well defined and might differ in different cultures and settings. The main objective of this systematic review is to summarize literature on the topic and propose a definition that might help aging research and practice in the future.Eight databases including PubMed, Scopus, CINAHL, Web of Science, EMBASE, PsycINFO, Proquest, and Age Line bibliographic will be run individually to retrieve relevant literature on loneliness among elderly population using subject headings and appropriate MeSH terms. Inclusion and exclusion criteria will be developed and refined by the research team. Two reviewers will participate in each search stage including abstract/title and full text screening, data extraction, and appraisal. We will restrict our search to articles published in the English language biomedical journal between 2000 and 2017. The protocol adheres to the standards recommended by the PRISMA-P.The results of this systematic review can present a more accurate definition of loneliness for researchers who aim at conducting new primary and secondary studies on this subject.CRD42017058729.