Saint Maximus the Confessor says,
"Dispassion is a peaceful condition of the soul."Saint Isaac the Syrian says,
"Dispassion doesn't mean to no longer feel the passions, but to no longer accept them."Diadochos of Photiki says,
"Dispassion doesn't mean to be no longer be attacked by demons..., but being attacked by them, to remain unconquered."
Saint John Climacus says dispassion is,
"the heaven in the heart of the mind, which considers the cunning of the demons as just toys."Fr.Dimitru Staniloae summarizes,
So dispassion would be that state of the soul in which it defeats every temptation.This state is attained after much ascetic work and is a most positive strength capable of defeating every passion. It can also be seen as the possession of all the virtues.
Reference: Orthodox Spirituality, pp 185 - 187