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Informal green spaces (IGSs) are vital yet under-researched urban areas that enhance biodiversity, provide ecosystem services, and improve the well-being of urban residents. However, the lack of a consistent definition and comprehensive understanding of their multifunctional roles has hindered their effective integration into urban planning. The current literature review aimed to clarify the concept of IGSs, analyze research trends, and identify further research areas. Using a combined bibliometric and systematic analysis approach, 150 articles from the Web of Science database, published from 1996 to 2024, were analyzed. The systematic analysis identified 54 relevant documents on the effects of green areas, revealing a diverse and growing body of research on IGSs, including their types, distribution, and socioeconomic contexts. The findings indicated an increasing trend in collaborative studies, using "informal green space" as the official term. This review proposed a clear and comprehensive definition of IGS, emphasizing its visibility, lack of formal recognition, minimal management, spontaneous vegetation, and temporary nature and underscoring its substantial environmental and social benefits. Furthermore, this review highlighted the need for standardized definitions and interdisciplinary studies to fully harness the potential of IGSs, thereby emphasizing their essential contribution to urban biodiversity and the regulation of urban microclimates. [ABSTRACT FROM AUTHOR]