not complete glossary, or even necessarily completely accurate to how original coiners use these words. just personal interpretation and use here.

basics

alterhuman, alterhumanity	experiences and identities which not entirely typically human. this can include animals, other creatures, characters, objects, and more. shadow's welsh equivalent term is noliddyn.
plural system	many people who share one body.
shift	temporary change in behaviour, self-perception, or otherwise that come from alterhumanity. shadow's welsh equivalent term is newid, which in general means change. jump to shifts for more.
therian, therianthrope	someone with identity, in part, like animal or other creature. shadow's welsh equivalent term is rhanifail.

shifts

envisage shift:	shift where physical self perception feel in line with nonhuman mien, without necessarily feel phantom limbs. not physical body change, more like mental map. this can mean that expect see in mirror, expect others see this way, or for moment forget that not have same abilities and limbs. unlike phantom shifts, no control over limbs - only exist as vague impression. shadow's welsh equivalent term is newid wyneb.
mental shift:	to take on nonhuman mental characteristics or urges. for example, cat alterhuman may suddenly crave fish, or feel urge to pounce on something. shadow's welsh equivalent term is newid meddyliol.
phantom shift:	to feel limbs or features that not exist on physical body, like wings or tails. not necessarily come with envisage shift. may have mental control over limbs, or feel some sensation - with limits. shadow's welsh equivalent term is newid rhith.
physical shift:	not term that identify with, only here for summary. to physically shapeshift back and forth as nonhuman. term and community that most often use by abusers and their victims. for discussion purposes, shadow's welsh equivalent term is newid corfforol.
sensory shift:	somatic shift subtype. shift where feel as if senses become more nonhuman. not gain super senses, but do think in these times genuinely process and prioritise sensory information different. all still within human limits, but feel different than usual. shadow's welsh equivalent term is newid synhwyraidd.
somatic shift, psychosomatic shift:	shifts with effect on physical body - within typical limits, so, not physical shift aches without medical reason and clear relation to alterhumanity, such as ache where wings should be sensory changes, as above shadow's welsh equivalent term is newid somatig or newid seicosomatig.