What I mean is that there is no universal definition of which properties are safety properties. In principle, you can define any property you can formally reason about as a safety property. Therefore, whenever you talk about safety, you first have to define which properties you mean by that.
In the context of Rust, there are a number of safety properties that Rust guarantees (modulo unsafe, FFI UB, etc.), but that set of safety properties is specific to Rust and not universal. For example, Java has a different set of safety properties, e.g. its memory model gives stronger guarantees than Rust’s.
Therefore, the meaning of “language X is safe” is entirely dependent on the specific language, and can only be understood by explicitly specifying its safety properties.
That's true for "soundness" too. Things aren't just "sound". They are sound with respect to something. So when you use "soundness" as a comparison against "safety", you'll have to understand how somebody could interpret your post in the way that I did.
Almost all discussion about Rust is in comparison to C and C++, by far the dominant languages for developing native applications. C and C++ are famously neither type-safe nor memory-safe and it becomes a pretty easy shorthand in discussions of Rust for "safety" to refer to these properties.
In the context of Rust, there are a number of safety properties that Rust guarantees (modulo unsafe, FFI UB, etc.), but that set of safety properties is specific to Rust and not universal. For example, Java has a different set of safety properties, e.g. its memory model gives stronger guarantees than Rust’s.
Therefore, the meaning of “language X is safe” is entirely dependent on the specific language, and can only be understood by explicitly specifying its safety properties.