The word Immutable means “cannot be changed.” And ledger is a fancy term for record, a record of something. Therefore an Immutable Ledger is a record that cannot be changed.

In the digital age we need data security and proof that the data has not been altered -- that’s the only way we can trust the digital data.

Such trust and proof of trust is very necessary when we are tracking transactions of money. Imagine if you sent me an electronic funds transfer of $1000 without any proof that you sent it and no way to verify that you sent it. I would not believe you until the money showed up, and if it didn’t show up--what then?

All banks and credit card processing systems use some sort of ledger to keep track of all the transactions that happen. But what if we don’t want to trust a big corporation, bank, or government with our money? Who do we trust?

Blockchain technology introduced the Immutable Ledger. It’s based on math. You put your trust in that math, knowing no one can alter it or change it. 

But how does blockchain ensure immutability of the ledger? The foundation is what’s called the hash. The hash is like a digital signature and if a hacker tries to alter anything in the ledger, its hash will change. 

Once the hash changes and  no longer matches the previous hash in the ledger, the blockchain will reject that hash (making it null and void like a bad check). The hacker would have to change the next block, and the block after that, and basically the entire blockchain. 

They can’t do that because a copy of the blockchain resides on multiple computers around the world. The hacker would need to make all of these changes simultaneously -- hacking into every computer all around the world. This is quite literally impossible. And therefore the ledger becomes immutable -- unable to be changed.