Actually, you can reverse Bitcoin transaction before it gets confirmation but in reality, you do not reverse a transaction, you simply replace the transaction but when it comes to confirmation, the thing is that they become progressively harder to reverse but theoretically they are never impossible to reverse.
It's double spend on a transaction that has yet been confirmed by Bitcoin miners and relayed by Bitcoin nodes. Reversing transactions is about transactions that already got at least one confirmation.
There is a reason companies are waiting for 6 confirmations at least. A transaction with one confirmation can potentially disappear during a blockchain reorganization.
Also, Bitcoin transaction is not verified by the receiver but verified by independent nodes and then included by miners in blocks.
This explanation in "Mastering Bitcoin" book can give more insights about that and easily to understand. Because of more vulnerability of first blocks like first one, first three or six ones, centralized exchanges usually require 1 to 3 confirmations before either crediting deposit to user account or allowing users to withdraw funds after their deposits.
Chapter 11: The blockchain.One way to think about the blockchain is like layers in a geological formation, or glacier core sample. The surface layers might change with the seasons, or even be blown away before they have time to settle. But once you go a few inches deep, geological layers become more and more stable. By the time you look a few hundred feet down, you are looking at a snapshot of the past that has remained undisturbed for millions of years.
In the blockchain, the most recent few blocks might be revised if there is a chain reorganization due to a fork.
The top six blocks are like a few inches of topsoil. But once you go more deeply into the blockchain, beyond six blocks, blocks are less and less likely to change.
After 100 blocks back there is so much stability that the coinbase transactionthe transaction containing the reward in bitcoin for creating a new blockcan be spent. While the protocol always allows a chain to be undone by a longer chain and while the possibility of any block being reversed always exists, the probability of such an event decreases as time passes until it becomes infinitesimal.
