apodictic
Concise Oxford English Dictionary © 2008 Oxford University Press:
apodictic /ˌapəˈdɪktɪk/ (also apodeictic /-ˈdʌɪktɪk/)
▶adjective formal clearly established or beyond dispute.
– origin C17: via L. from Gk apodeiktikos, from apodeiknunai ‘show off, demonstrate’.
'apodictic' also found in these Oxford entries:

