PRISMA
Short definition
PRISMA is an international guideline setting out what a systematic review or meta-analysis should report, including a checklist and a flow diagram of study selection.
In plain language
The current version, PRISMA 2020, has a 27-item checklist and a flow diagram showing how many studies were found, excluded and included, and why. It does not measure the quality of the review, but it makes it transparent and reproducible. Many scientific journals require it; there are extensions, such as PRISMA-ScR for scoping reviews.