-sis

Ultimately from Ancient Greek -σις (-sis, “forms noun of action”), often via Latin or French. Identical in meaning with Latin -entia, English -ing.

-sis (plural -ses)

1. forming nouns of action or process

   catharsis, dialysis, diagnosis, crisis

2. (medicine) forming nouns of condition

   psoriasis, sepsis, tuberculosis, psychosis

3. forming additional nouns

   basis, oasis, thesis, stasis

Not very productive: primarily used for borrowed terms from Ancient Greek, though there are also modern coinages based on Ancient Greek roots.

• ISS, SSI

-sīs

1. dative/ablative masculine/feminine/neuter plural of -sus

-sis

1. -sis

• “-sis”, in Diccionario de la lengua española, Vigésima tercera edición, Real Academia Española, 2014