The term "Open Source" is very clearly defined: https://opensource.org/osd
Unless you have some reason to assume that somebody is using this term mistakenly (which may happen with any term), there's no problem with it.
Actually, "copyleft or public domain" is a bad term, because it would exclude things like Godot Engine, which is neither copyleft nor public domain (it's MIT). Don't use terms like that, use the clearly defined ones like "open source", "free software" or "FLOSS" - they all mean the same.