WebAssembly is used in Polkadot and Substrate as the compilation target for the runtime.
What is WebAssembly?
WebAssembly, shortened to Wasm, is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for the compilation of high-level languages like C/C++/Rust, enabling deployment on the web for client and server applications.
WebAssembly is a platform-agnostic binary format, meaning it will run the exact instructions across whatever machine it operates on. Blockchains need determinacy to have reliable state transition updates across all nodes in the peer-to-peer network without forcing every peer to run the same hardware. Wasm is an excellent fit for reliability among the diverse set of machines. Wasm is both efficient and fast. The efficiency means that it can be uploaded onto the chain as a blob of code without causing too much state bloat while keeping its ability to execute at near-native speeds.