@TheCharlatan has been heading up the effort to put all the consensus code from Bitcoin Core (and only the consensus code) in one place. It's called libbitcoin kernel (not connected to the libbitcoin project led by Erik Voskuil). Today, they released a C header API.

When Satoshi originally released code for Bitcoin, everything was all mixed together like spaghetti: node, wallet, mining software, peer to peer network, transaction relay. Not all of these various tools affected consensus or even needed to know very much about it.

Bitcoin Core is a direct descendant of the original Satoshi client, and over time the project has been attempting to separate out its different aspects. One important part of this process is separating all the code that deals with consensus from everything else and then figuring out how things like mining software or wallets or mempool software can ask for information from the part that implements consensus rules.

But the most exciting part of this project is that it should allow more customization among node implementations based on consensus code in Bitcoin Core.

Also: a lot of people worked very hard on this and it's a super impressive achievement.