simultaneous contemplation of the threefold truth
A Tendai Buddhist meditation practice in which one contemplates the three truths (emptiness, provisional existence, and the middle way) simultaneously in a single moment of mind.
See also: 三諦
Isshin-sangan is an important meditation method of the Tendai school.
三諦 refers to the three truths themselves (emptiness, provisional existence, middle way), while 一心三観 is the practice of contemplating them simultaneously.
From 一心 (one mind) + 三観 (three contemplations). A technical term in Tendai Buddhism.