user52911: I've read some articles about the bitcoin network, some of them say it is necessary for a transaction, before adding to blockchain, to be validated by more than half of all nodes in bitcoin network. Is my understanding true? If yes, is it true to say that after exactly 51% of nodes have validated the transaction, it will be added to blockchain?

user48884: No, it is not necessary for any node validate a transaction before it is included in a block and thus added to the blockchain. A transaction can be included in a block without being validated, but doing so risks the transaction being invalid and thus making the block invalid. This has happened several times in the past; miners will make their own transaction and include it in their own block without broadcasting it to the rest of the network. 

user24974: It is not the percentage of nodes that matters, but the percentage of total network hashing power.  If 51% of the network's hashing power is mining on top of a block, then you could say the network considers all the transactions in that block to be valid, and accepted.  Other nodes will continue to validate the transactions independently, but that is so they know whether or not to trust the blocks that the miners propagate.  It allows them to be trustless.
So what happens when 51% of hashing power is mining on top of a block containing a transaction that 49% of the network consider to be invalid?  This is what a hard fork is.  It means the 49% will continue mining on what they consider to be the last valid block.

52911: <p>I've read some articles about the bitcoin network, some of them say it is necessary for a transaction, before adding to blockchain, to be validated by more than half of all nodes in bitcoin network. Is my understanding true? If yes, is it true to say that after exactly 51% of nodes have validated the transaction, it will be added to blockchain?</p>
 48884: <p>No, it is not necessary for any node validate a transaction before it is included in a block and thus added to the blockchain. A transaction can be included in a block without being validated, but doing so risks the transaction being invalid and thus making the block invalid. This has happened several times in the past; miners will make their own transaction and include it in their own block without broadcasting it to the rest of the network. </p>
 24974: <p>It is not the percentage of nodes that matters, but the percentage of total network hashing power.  If 51% of the network's hashing power is mining on top of a block, then you could say the network considers all the transactions in that block to be valid, and accepted.  Other nodes will continue to validate the transactions independently, but that is so they know whether or not to trust the blocks that the miners propagate.  It allows them to be trustless.</p>

<p>So what happens when 51% of hashing power is mining on top of a block containing a transaction that 49% of the network consider to be invalid?  This is what a hard fork is.  It means the 49% will continue mining on what they consider to be the last valid block.</p>
