St. John of the Cross writes in The Dark Night (BK I, Ch 10) that "contemplation is nothing else than a secret and peaceful and loving inflow of God, which, if not hampered, fires the soul in the spirit of love".
St. Teresa of Jesus describes contemplation in The Way of Perfection (Ch 25) as:
"The soul understands that without the noise of words this divine Master is teaching it by suspending its faculties, for if they were to be at work they would do harm rather than bring benefit. They are enjoying without understanding how they are enjoying. The soul is being enkindled in love, and it doesn't understand how it loves. It knows that it enjoys what it loves, but it doesn't know how. It clearly understands that this joy is not a joy the intellect obtains merely through desire. The will is enkindled without understanding how. But as soon as it can understand something, it sees that this good cannot be merited or gained through all the trials one can suffer on earth. This good is a gift from the Lord of earth and heaven, who in sum, gives according to who He is . What I have described, daughters, is perfect contemplation."