Dictionary Definition :
Definition[1]
:
An esoteric objection to three, six, or nine persons worshipping vtogether.
Source
:
A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous
Definition[2]
:
(雜語)密法不許三六九人同入灌頂壇。大日經疏四曰:「一曼荼羅中不得同時為三人六人九人灌頂,蓋如來密意,阿闍梨不釋所由。」
Source
:
丁福保《佛學大辭典》