Mid 17th century (in an earlier sense). From post-classical Latin phasis or its etymon ancient Greek ϕάσις appearance, in Hellenistic Greek also phase from ϕα-, ablaut variant of the stem of ϕαίνειν to bring to light, cause to appear, show + -σις.
Definition of phasis in US English:
phasis
noun
rare
The first appearance of the new moon.
Origin
Mid 17th century (in an earlier sense). From post-classical Latin phasis or its etymon ancient Greek ϕάσις appearance, in Hellenistic Greek also phase from ϕα-, ablaut variant of the stem of ϕαίνειν to bring to light, cause to appear, show + -σις.