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"distro" is short for "distribution of software", not necessarily an OS etc., just some collection of software. Of course it is very handy to have everything that works together in one collection including OS, utilities, services and apps. UNIX was shared about on magnetic tapes as a distribution of software. BSD is Berkley Software Distribution. The term lately is associated with GNU/Linux because Linux is hot, not because distros are distributions of Linux and software that goes with Linux. Then there is the distinction between source code and binary. A binary distribution is useful when someone has bothered to compile everything in the environment that is to run on the target. Source could work on any hardware by building from source or doing some porting.

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