Lund, H. & Christensen, R. (2016). "a systematic review is a structured and preplanned synthesis of original studies that consists of predefined research questions, inclusion criteria, search methods, selection procedures, quality assessment, data extraction, and data analysis. No original research study should be deliberately excluded without explanation, and the results from each study should justify the conclusion" Lund, H., Juhl, C. & Christensen, R. (2016). Systematic reviews and research waste. The Lancet, 387(10014), 123-124. https://doi.org/10.1016/s0140-6736(15)01354-9
Grant, M. J., & Booth, A. (2009). A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information And Libraries Journal, 26(2), 91-108. https://doi.org/10.1111/j.1471-1842.2009.00848.x
..."what exactly is a systematic review? How does it differ from other types of reviews? And how do you systematically search the literature?" Karolinska Institutet, University library
Useful guide from Karolinska Institute Library (Sweden): Systematic reviews