I have a different preferred starting point which is less descriptive but more operational: WebAssembly is a new fundamental abstraction boundary.

But WebAssembly is a new point in this space. Unlike the Linux ABI, there is no fixed set of syscalls: WebAssembly imports are named, typed, and without pre-defined meaning, more like the C ABI. Unlike the C ABI, WebAssembly modules have only the shared state that they are given; neither side has a license to access all of the memory in the “process”. And unlike HTTP, WebAssembly modules are “in the room” with their hosts: close enough that hosts can allow themselves the luxury of synchronous function calls, and to allow WebAssembly modules to synchronously call back into their hosts.