## 3 Understanding Non-Fungible Tokens  
### Definition of NFTs and Emergence  
Combining and extending the definitions of [PERSON] and [PERSON] [35], [PERSON] et al. [40] and [PERSON] [41], we define a non-fungible token (NFT) as a _cryptographically unique, indivisible, irreplaceable and verifiable token that represents a given asset, be it digital, or physical, on a blockchain_. At present the vast majority of NFTs are built on the Ethereum blockchain network [41] and are therefore Ethereum tokens; it is worth noting, however, that emerging blockchain platforms such as Flow [42], Tezos [43] and Algorand [44] have also been adding support for NFTs. According to Ethereum's founder [PERSON], his main motivation for developing Ethereum was to explore \"the blockchain concept can be used for more than just money\" [45]. As a result, the fundamental difference between Ethereum and the Bitcoin blockchain network is that an Ethereum token (i.e., a token stored and traded on the Ethereum blockchain) is created and managed by a so-called \"smart contract\", which allows for programmatic computation, notably supporting \"all types of computations\" [46]. [PERSON] describes smart contracts as \"systems which automatically move digital assets according to arbitrary pre-specified rules\" [46]. In other words, a smart contract can be described as a self-executing contract between two parties, whose terms of agreement are written into lines of code and whose execution and related transactions are trackable, irreversible and exclusively controlled by code [47]. The concept of NFTs was first introduced in 2012 with Bitcoin's \"Colored Coins\", which referred to tokens that represent any type of physical asset, such as real estate properties, cars and bonds [48]. In their current format, NFTs were implemented in 2017, when [PERSON], a contributor to the Ethereum source code repository and founder of the digital collectibles game CryptoKitties, introduced the ERC-721 smart contract standard, which allowed for the creation of a new type of Ethereum tokens [49]. [PERSON] explained the standard would allow for the implementation of \"non-fungible tokens\", also \"referred to as 'NFTs\", providing \"basic functionality to track and transfer ownership of NFTs\" [49] on the Ethereum blockchain. Up until then, for purposes of interoperability, the vast majority of Ethereum tokens were based on the ERC-20 protocol, whose defining feature is that they are fungible, i.e., replaceable and interchangeable [50]. For example, on Ethereum, all cryptocurrencies (except for Ether, which is Ethereum's so-called native cryptocurrency) are created using an ERC-20 contract for that very reason; similar to a real-world currency, where a bank note is fungible and interchangeable with every other bank note of the same value, each ERC-20 token is fungible and interchangeable with every other token of the same type. By introducing the ERC-721 token protocol, [PERSON] offered an alternative to that. Other smart contract protocols for creating NFTs have also been proposed. Although ERC-721 is considered the \"gold standard\" [51], the ERC-1155 \"multi-token standard\" [52] offers greater flexibility in the creation and management of NFTs, by offering support for batch operations, reducing gas fees and subsequently carbon emissions by 90% [53]. Another standard worth noting is EIP-2981, which enables universal support for royalty payments in all NFT marketplaces and ecosystems [54]. EIP-2981 addresses the lack of standardisation and interoperability across ecosystems, when it comes to the enforcement of resale rights across different platforms. Finally, fractionalised NFTs (F-NFTs), which support the fractional ownership of NFTs, are expected to be increasingly adopted, especially for high-value collectibles [55]. It could be argued that Ethereum's edge over other blockchains is primarily due to market domination.