[adwn]

> For example, the copy on write (COW) mechanism of unix where memory pages are mapped to the same location until a forked process writes to one

That is shared memory – memory which is shared between two or more threads or processes.

> There's also Redux and Closure's immutable state tree that makes copies under a similar mechanism to COW but through code

That's also shared memory.

"Shared memory" does not necessarily mean "shared mutable memory". Rust, for example, statically prevents memory from being simultaneously shared and mutable (except through synchronization primitives like mutexes etc.). A pure actor-based language, in contrast, would prevent even immutable memory from being shared.