Proof of inclusion is cryptographic evidence that a specific certificate, log entry, or other record is part of a published Merkle tree. Instead of trusting a claim on its own, a verifier checks a short path of hashes from the item to the tree root. If the path matches the published root, the item is proven to be included.
In cyber security, this matters because it makes public logging and transparency efficient. Certificate systems can use proof of inclusion to show that a TLS certificate was actually recorded in a transparency log or Merkle Tree Certificate set without sending the whole dataset. Defenders use these proofs to detect hidden or misissued certificates, while attackers would have a harder time forging a valid inclusion claim because changing any node breaks the hash chain. In practice, the proof is a compact control that helps web trust scale while keeping verification fast.