Cryptographic boundary
The explicitly defined perimeter in FIPS 140 that determines what belongs to a cryptographic module — and therefore what the certificate actually covers.
Everything inside the boundary — hardware, firmware, software — is what has been reviewed and validated. Keys and other critical security parameters may only cross it through controlled paths, in plaintext only under strict conditions, and the module must be able to zeroise them. For an HSM the boundary is often literally the metal enclosure with its tamper protection; for a software library it can be a single binary, such as OpenSSL's FIPS provider.
The boundary explains much of the confusion around "FIPS validated". A product can contain a validated module without being validated itself, and a validated binary stops being validated if you rebuild it with a different flag. Where the line is drawn is spelled out in the module's security policy, a document few people read and everyone should.