In the context of systematic reviews and meta-analyses, a protocol is a document that presents an explicit plan for a systematic review. The protocol details the rationale and a priori methodological and analytical approach of the review.
"The systematic review protocol is a detailed description of the objectives and methods of the review. The protocol should include information regarding the context and rationale for the review, primary outcomes of interest, search strategy, inclusion/exclusion criteria, data synthesis strategy, other aspects of the research plan."
For more information, see page #72 of Chapter 2: "Standards for Initiating a Systematic Review." Institute of Medicine. Finding What Works in Health Care: Standards for Systematic Reviews. Washington, DC: The National Academies Press, 2011. doi:10.17226/13059.
The PRISMA-P checklist contains 17 items considered to be essential and minimum components of a systematic review or meta-analysis protocol. The final PRISMA-P 2015 checklist contains 17 numbered items (26 including sub-items) Items are categorized into three main sections: administrative information, introduction, and methods (Table 3).
This article also outlines items that should be included in a protocol:
Moher D et al. "Preferred reporting items for systematic and meta-analysis protocols(PRISMA-P) 2015 statement. Sys Rev. 2015 Jan 1;4:1. http://systematicreviewsjournal.biomedcentral.com/articles/10.1186/2046-4053-4-1.
If you need more information about each section, see this article here:
Shamseer L, et al. Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015: elaboration and explanation. BMJ. 2015 Jan 2;349:7647. doi: 10.1136/bmj.g7647. http://www.bmj.com/content/349/bmj.g7647
You can find examples of protocols in the PROSPERO database.
You can type in any topic in the search box and should be provided a list of protocols on that topic.