Solid (derived from "SOcial LInked Data") is a proposed set of conventions and tools for building decentralized social applications based on Linked Data principles (LDP).
Solid is a set of [modular and extensible specifications][solid-specs], which build on, and extend the founding technology of the world wide web (HTTP, REST, HTML). They are 100% backwards compatible with the existing web. Each spec, taken in isolation, provides extra features to an existing system. However, when used in combination, they enable exciting new possibilities for web sites and applications.
The key principles at the base of this set of conventions - it's worth to repeat that concept to avoid any ambiguity - are basically 3:
1. True data ownership
Users should have the freedom to choose: