[FabHK]

It's a block chain. From the article:

> a copy of that seal and every other seal created by Surety’s customers is sent to the AbsoluteProof “universal registry database,” which is a “hash-chain” composed entirely of Surety customer seals. This creates an immutable record of all the Surety seals ever produced, so that it is impossible for the company or any malicious actor to modify a seal.

Another well known example of course is git (which slightly extends the definition to a directed acyclic graph).