This smart contract is a piece of code that runs on ethereum's blockchain and as such is by definition decentralized. It is a self executing contracts with the terms of the agreement between buyer and seller being directly into line of code. It does not care about who wrong whom or what your name is.Once it live on ethereum's blockchain, no one can alter it.
A smart contract does not explicitly have to be based on ethereums blockchain. Neither does it have to be decentralized.
Ethereum is not the only platform which introduced smart contracts on the blockchain. But it is the most accepted and adopted.
Smart contracts (in future) could also be handled by a blockchain which is not completely decentralized. Licences, for example, could be easily
handled via smart contracts on a (non-public) "centralized blockchain".