It would be dishonest to say there is a perfect definition of the metaverse, but the easiest way to visualize it would likely be to use the book (and movie) 'Ready Player One' as an example. In the story, most of humanity interacts through a digital world, not only for games and entertainment, but for their jobs, education, etc.
The metaverse, in essence, is an active digital world where one could accomplish pretty much anything that can be done in the physical world. Your identity in such a world would be your avatar, much like how your body in the physical world is, well, you.
By using and engaging with these companies' products, you are giving them data, which they use to generate revenue one way or another, usually through advertising. If that product is an entire world of their own, well, you see where this is going; that's a lot of data. There is one simple solution to avoid this dystopian future where a megacorporation controls all your data, and that is decentralization.
How Can DeFi Help?
By decentralizing the metaverse, there is an immediate shift in the way data is handled. In decentralized finance and web 3.0 technologies in general, your data is under your control instead of in a centralized server owned by some entity. Not only that, but by decentralizing the control over this new world, you are moving the incentive to contribute to the ecosystem, instead of controlling its infrastructure and data. Companies and/or individuals that provide the best products or services within the metaverse will be rewarded by doing so, through the world's economy.
Speaking of which, any fully fledged metaverse will needs its own economy, much like how the physical world functions. Attributing value to assets and facilitating asset transactions are issues DeFi inherently solves. A centralized entity would never be able to rival the ability with which decentralization can provide security and transparency to the metaverse's economy, let alone implement better governance systems for overseeing it.
Digital identities are also as aspect DeFi can contribute to. Your address on the blockchain is already inherently a form of identity; a very secure one at that. Certainly more secure than anything implemented by centralized solutions. Furthermore, through ERC-721 contracts (NFTs), having unique and verifiable digital avatars to represent you in this new world has never been easier.
Although these are the main points to cover, there are many aspects of decentralization that would aid the creation of the multiverse such as having verifiable interactions between users, being able to decentralize storage to a scale almost impossible for any one centralized entity, and more.
What's Being Built
Most of the "metaverses" built so far are geared towards gaming. Even still, the underlying metaverse concepts are there - fully decentralized, identities via NFTs, interacting with other users on-chain, transacting assets, etc.
More ambitious projects however, such as Decentraland and Enjin aim to more fully realize the potential of the metaverse, and many similar projects are being built on Avalanche. Examples of such could be Imperium Empires focusing on its space theme, while Kalao focuses on digital art and creating awesome VR experiences.
We can't be sure what the final form of the metaverse will look like, but one fact is certain; we'd be headed straight for a dystopian future were it not for decentralized technologies such as DeFi. Whether it is through the projects being built now, or through some other means we haven't even begun exploring, the metaverse will bring major change to the way we interact with each other in the digital realm.