Ghost refers to the ethereal self of the soul of a particular biological or synthetic entity. Ghosting is the act of projecting one’s Ghost into another entity or being to assume control over the said entity. The physical body or host is nothing more than a vessel for hosting an entity’s Ghost.
This is how Oppenheimer can ghost himself into a completely different body or vessel. This is also referred to as shape-shifting, like the AlienPede, where the Ghost can hijack and imitate another entity’s vessel.
Furthermore, synthetic or non-biological entities can project themselves into other entities’ vessels, which is one of the features of the Order’s Octa-Mind technology.