Whether in 2D, 2.5D [[[A set of imaging techniques and technologies halfway between 2D and 3D.]]] or 3D, a "virtual world" can be considered as a "persistent, multi-user digital environment, accessible via the internet and in which users interact with it and with other users through an avatar, their graphical representation on the screen” [[[Lucas J.-F. (2018), « Les figures de l’habitant dans les mondes virtuels », Sciences du jeu, 10 | 2018: https://journals.openedition.org/sdj/1353]]]. Consequently, when digitally simulated environments are neither persistent nor multi-user, as is the case for many video games, they are certainly "simulated" or "virtual" environments, but not "virtual worlds".
The Metaverse's promise would therefore be to interconnect a multitude of virtual environments (whether virtual worlds or simulated environments of all kinds) based on fundamental building blocks.

As Philippe Fuchs suggests, we need to distinguish between virtual reality (VR), which "should enable a user to act physically in an artificial environment", and the systems (headsets, glasses, CAVEs [[[A CAVE is an "automatic virtual environment", which is usually closed. It is similar to a "cave". Generally, "screen walls" surround one or more users who are placed at the centre of the CAVE structure. To enhance perceptual immersion, people wear equipment such as 3D helmets and glasses, or suits and gloves to capture movements.]]]) that enable this "acting”39. According to this definition, VR can include any simulated environment or any virtual world as long as a user is immersed in it and can act within it. This distinction is important, because virtual reality was built on this rapprochement between simulated environments and the systems, interfaces, and devices that enable users to act in them; since it is precisely the latter that reinforce "the fact of believing what does not exist", which for Coiffet is the main objective of virtual reality [[[Coiffet P. (1995), Mondes imaginaires, les arcanes de la réalité virtuelle, Paris: Hermès, p. 31.]]].
As a result, Fuchs correctly points out that sensorimotor experiences in virtual reality or in metaverses "will share some similarities", although there are some notable distinctions: the Metaverse is or allows "a very large number of users simultaneously; [a] persistent and ever-evolving artificial world; accessible from anywhere; commercial transactions”.

If current discussions on the Metaverse are often mistaken for those on Web3 (sometimes written in lower case depending on the authors' considerations), it is mainly because they set out the terms of a debate that questions both digital services and their business models, as well as the players behind them. Metaverse, Web3, blockchain, and cryptocurrencies encompass various concepts, technical architectures, technologies, principles, and values that are not consubstantial. Moreover, many discussions attempt to distinguish between Web3 and Web 3.0, with arguments that are not always the same. Our objective here is not to offer exhaustive definitions of these terms, but rather to illustrate their differences and underscore that they are not specific to the Metaverse.
Web 1.0 is about the interconnection of digital resources through hyperlinks. It is a "consultation" web, where most users visit websites produced by experts or organisations (or on their behalf). Web 2.0, on the other hand, represents the web of interactions, content creation, and sharing (text, images, videos, audio, etc.) by users themselves (referred to as User Generated Content or UGC), as well as their online participation. It witnessed the proliferation of social media platforms.
There is subsequently some confusion as to what Web 3.0 is, would be, or could be. On the one hand, some consider that "Web 3.0, also known as Web3, is the third generation of the World Wide Web [and is] designed to be decentralised, open to everyone (with a bottom-up design), and built on top of blockchain technologies and developments in the Semantic Web, which describes the web as a network of meaningfully linked data”. On the other hand, some suggest that Web 3.0 should be differentiated from Web3, considering that "the term 'Web3' is different from the concept of 'Web 3.0', which refers to the Semantic Web, theorised by Tim Berners-Lee, the founding father of the World Wide Web. The concept of Web3 was first used by Gavin Wood [[[Gavin Wood is a computer scientist and founder of the Euthereum cryptocurrency.]]] in 2014, referring to a 'decentralised online ecosystem based on blockchain'. The idea of a Web3 gained popularity towards the end of 2021, largely due to interest from cryptocurrency enthusiasts and large investors” [[[Galienni S., Truphème S. (2023), La vague Web3. Maîtriser les nouveaux codes du marketing 3.0. Blockchain, NFT, DAO, Métavers…, Dunod, p. 21.]]].

While Web 3.0 focuses on enhancing the semantic understanding of data to provide users with an improved internet experience, Web3 denotes a decentralized internet, sometimes reminiscent of the early utopias of the internet and the web. It positions itself as an alternative to the major Web 2.0 platforms’ business models. According to Matthew Ball, Web3 represents a somewhat “ill-defined Internet of the future, one built around users and independent developers rather than the dominant aggregation platforms like Google, Apple, Microsoft, Amazon, and Facebook. It represents a more decentralized version of today's Internet, with many believing it would be better served by blockchain technology”. The prospects for Web 3.0 and Web3 aren't inherently incompatible, but they pursue different objectives.

If that was all it took to muddy the waters, some, like the European Commission, have taken the liberty of outlining the contours of a Web 4.0 (spelled randomly, with or without capitalisation, depending on the document) that will "allow an integration between digital and real objects and environments, and enhanced interactions between humans and machines”.

"A digital twin is a dynamic digital model of an object (car, building, city, etc.), a process (production, training, etc.) or a physical or biological system (human body), fed by data from the corresponding real-world model over its entire life cycle” [[[Lucas J.-F. (2022a), « Imaginaires et traditions des jumeaux numériques urbains », in P. Musso (dir.), La Renaissance Industrielle, Paris : Manucius, pp. 70-80. ]]]. In short, "a digital twin is neither the environment or the 3D model, nor the database that feeds it or an activity, nor the physical benchmark model: it is the combination of all three” [[[Lucas J.-F. (2022b), « Traditions et imaginaires des jumeaux numériques urbains », presentation given to l’Institut des hautes études pour la science et la technologie (IHEST), 2021-2022 National training programme “Mobilising resources for transitions, transformations, breakthroughs, and metamorphoses”, 10 February 2022.]]].
This type of device can be used, for example, to simulate behaviour, optimise processes and yields (in industry, in a factory, in agriculture, etc.), monitor infrastructure, equipment, urban and natural environments, perform predictive maintenance or simulate crisis scenarios (pollution spikes, fires, floods, etc.). Digital twins are increasingly being used in a wide range of fields (design, training, health, etc.), using either a screen or a virtual reality headset.
For some, digital twins are considered precursors of an industrial metaverse, or even a Business to Business (BtoB or B2B) [[[Refers to commercial relationships in which one company carries out a commercial transaction with another. BtoB is sometimes referred to as "trade market "]]] (on this subject, see “BtoC and BtoB”) Metaverse that has existed for decades. However, it is important to clarify from the outset that certain fundamental elements at the heart of the Metaverse definition cannot be compared with the reality, challenges, and objectives of digital twins. Whether we are talking about industrial digital twins (aircraft, factories, etc.), digital twins relating to the human body, or digital twins specific to cities and regions, they do not address the same issues, problems, or objectives as those of the Metaverse.
If, like Building Information Modelling (BIM) or City Information Modelling (CIM), a city's digital twin needs to render a large number of interoperable data (mobility, air quality, various flows, etc.), software and applications (directly with each other or via APIs), it does not need to offer a shared, simultaneous, and persistent experience to millions of individuals; when these are the founding elements of the Metaverse definition. Conversely, the Metaverse is not intended to have a scientific basis, as is the case with digital twins. Finally, most simulated environments and other virtual worlds that are assimilated to the Metaverse generally offer the possibility of financial transactions, which digital twins do not.