A CA with one or more trusted certificates containing public keys that exist at the base of a tree of trust or as the strongest link in a chain of trust and upon which a Public Key Infrastructure is constructed. “Trust anchor” also refers to the certificate of this CA.
Sources:
NIST SP 800-152 under Trust anchor

  An authoritative entity represented by a public key and associated data (see RFC 5914).
Sources:
NIST SP 800-57 Part 2 Rev.1 under Trust anchor

  An established point of trust (usually based on the authority of some person, office, or organization) from which an entity begins the validation of an authorized process or authorized (signed) package. A "trust anchor" is sometimes defined as just a public key used for different purposes (e.g., validating a certification authority (CA), validating a signed software package or key, validating the process (or person) loading the signed software or key).
Sources:
CNSSI 4009-2015

  1. An authoritative entity for which trust is assumed. In a PKI, a trust anchor is a certification authority, which is represented by a certificate that is used to verify the signature on a certificate issued by that trust-anchor. The security of the validation process depends upon the authenticity and integrity of the trust anchor’s certificate. Trust anchor certificates are often distributed as self-signed certificates. 2. The self-signed public key certificate of a trusted CA.
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Trust anchor

  A public or symmetric key that is trusted because it is built directly into hardware or software or securely provisioned via out-of-band means rather than because it is vouched for by another trusted entity (e.g., in a public-key certificate). A trust anchor may have name or policy constraints that limit its scope.
Sources:
NIST SP 800-63-4 [