Comment by tom_alexander

That's more of a historical artifact. The BSDs started as just "BSD": a set of patches for AT&T Unix that were _distributed_ by Berkeley. Eventually the patches became complete enough to be an entire operating system. _Then_ the various BSDs that we know today (FreeBSD, OpenBSD, NetBSD, DragonflyBSD) all forked and became completely independent operating systems. For decades, FreeBSD's kernel and userland has been developed independently from the OpenBSD kernel and userland which is developed independently from NetBSD's kernel and userland, etc. You could not take an OpenBSD program and run it on FreeBSD. Even recompilation from source isn't necessarily enough since the BSDs support different syscalls.

They are completely independent operating systems with a distant shared history.

Whereas on Linux, the distros are taking a common Linux kernel source, and combining it with their choice of common userlands like GNU. Debian has the same kernel and GNU userland that Arch and Fedora use. You could take a program compiled for Debian and run it on Arch, which is common these days due to Docker where you're pulling another distro's userland and running it on your distro's kernel. That is how Linux distros are "distros" whereas the BSDs are independent operating systems.