web-of-trust
Definition
In cryptography, a web of trust is a concept used in PGP, GnuPG, and other OpenPGP
-compatible systems to establish the authenticity of the binding between a public key and its owner.
More
Its decentralized trust model is an alternative to the centralized trust model of a public key infrastructure (PKI
), which relies exclusively on a certificate authority (or a hierarchy of such). As with computer networks, there are many independent webs of trust, and any user (through their identity certificate) can be a part of, and a link between, multiple webs. The web of trust concept was first put forth by PGP creator Phil Zimmermann in 1992 in the manual for PGP.

More on Wikipedia