I always had issues with this sidecountry, backcountry, whatever country. I admit I'm from Europe, where we have piste (mostly groomed but as @Cheizz wrote, also few not groomed in each resort), which is controlled and off-piste, where you are on your own. 1m off piste, and you are on your own, so all these different terms for basically same thing is sort of confusing. But at least until now, I got impression that sidecountry is, as @cantunamunch said, "next to controlled areas". I agree this might be misleading, as there's nothing like being safe if it's next to controlled terrain, but on the other side, it still might be safer then pure backcountry. Not snow/avalanche vise, but general safety vise. For backcountry, I understand this as literally back country, middle of nowhere. No easy access, maybe with even no phone signal coverage. Side country would still in my mind have (relatively) easy access etc. So in case of accidents, it's easier and especially faster to get help.
But definitely, when talking about avi safety, side country is probably misleading, as it's, at least in my mind, exactly same as backcountry, even though it sounds "better" or "safer". No avi mitigation, no snow control, and you are completely on your own, regarding your decisions and your safety.
But definitely, when talking about avi safety, side country is probably misleading, as it's, at least in my mind, exactly same as backcountry, even though it sounds "better" or "safer". No avi mitigation, no snow control, and you are completely on your own, regarding your decisions and your safety.