From Russel's Mysticism and Logic
Metaphysics has been developed, from the first, by the union and conflict of two very different human impulses, the one urging men towards mysticism, the other urging them towards science. Some men have achieved greatness through one of these impulses alone, others through the other alone: in Hume, for example, the scientific impulse reigns quite unchecked, while in Blake a strong hostility to science co-exists with profound mystic insight.
He outlines the following characteristics of mysticism.
1 Revelatory Intuition
A Reality behind the world of appearance and utterly different from it... regarded with an admiration often amounting to worship; it is felt to be always and everywhere close at hand, thinly veiled by the shows of sense, ready, for the receptive mind, to shine in its glory even through the apparent folly and wickedness of Man.
The poet, the artist, and the lover are seekers after that glory: the haunting beauty that they pursue is the faint reflection of its sun. But the mystic lives in the full light of the vision: what others dimly seek he knows, with a knowledge beside which all other knowledge is ignorance.
Belief in unity, and its refusal to admit opposition or division anywhere... In Plato, this impulse is less prominent, being held in check by his theory of ideas; but it reappears, so far as his logic permits, in the doctrine of the primacy of the Good.
Following from the denial of all division there follows
2.1 Denial of time
2.2 Denial of the reality of evil
I've always had a low opinion of Russell in this regard and am happy to revise it.
(And thanks for asking this question!)
The main thing I would disagree with Russell is in the claim that the impulses to science and mysticism are in opposition. Complementary would have been better.