A permissionless technology is one that does not require permission to use. Most permissionless technologies are decentralized. For example, you don’t need permission to create software based on open source and open platforms, as well as open ecosystems such as PC software development. Many blockchains are permissionless.
In contrast, permissioned environments (such as the App Store, Steam, etc.) and traditional financial networks are gate-keepers that decide who is able to participate. They typically extract higher take rates for the privilege.
