Simplified Payment Verification. A feature of the Bitcoin protocol that enables nodes to verify payments without downloading the full blockchain. Instead, only the block headers need to be downloaded. This makes these wallets substantially easier to use. Every transaction must be present with all its parent and sibling hashes in a merkle tree up to the root. SPV client trusts the most difficult chain of block headers and can validate if the transaction indeed belongs to a certain block header. Since SPV does not validate all transactions, a 51% attack may not only cause a double spend but also make a completely invalid payment with bitcoins created from nowhere, However this kind of attack is very costly and probably to expensive.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc