Some transactions between PKI components will be performed through physical procedures rather than implemented electronically. Such transactions are described as out-of-band transactions.
NIST SP 800-15
Communication between parties utilizing a means or method that differs from the current method of communication (e.g., one party uses U.S. Postal Service mail to communicate with another party where current communication is occurring online).
NIST SP 800-32 under Out-of-Band
Used to refer to information transmitted through a separate communications channel.
NISTIR 7711 under Out Of Band