According to the Cochrane Handbook, a "systematic review attempts to collate all empirical evidence that fits pre-specified eligibility criteria in order to answer a specific research question". A systematic review also adheres to a specific methodology and often includes meta-analyses, wherein the collected data is combined using statistical methods.
Since they summarise the results from all original studies within a given field, systematic reviews are commonly regarded as high quality evidence. A pyramid is often used to visualise this hierarchy of evidence, with systematic reviews placed at the apex of the pyramid.