Best current practice
A Best Current Practice (BCP) means that a certain manner of proceeding is in general the most logical choice -- a de facto standard of sorts. This expression is often used in the context of computer programs and their implementation, as well as that of network protocols and their specifications.
A Best Current Practice is only a noncommittal suggestion, as is to be proceeded in a certain case. It is thus more flexible than a standard: As requirements and conditions change, from time to time, another manner of proceeding can be more promising, and subsequently implemented. Should the requirements permanently change, a BCP should be revised.
BCP is also the name of a numbered document series published by the Internet Engineering Task Force. All documents in this series are also Requests for Comments (RFCs); whereas a given RFC number always refers to a specific version of a document, a BCP number refers to the most recent revision of the document. (Thus, citations often reference both a BCP number and an RFC number.) BCPs are used to document guidelines, processes, methods, and other matters not suitable for standardization; most notably, the Internet standards process itself is defined in a series of BCPs, as is the formal organizational structure of the IETF, Internet Engineering Steering Group, Internet Architecture Board, and other groups involved in that process. (Contrast this with the STD document series, which defines the fully standardized network protocols of the Internet, such as IP, TCP, and DNS.)