From What is PrelestĚ?:
The term prelest is a Russian word which has come into Englisg usage for lack of a precise equivalent, although it is often translated as “spiritual delusion ,” “spiritual deception,” or “illusion,” accepting a delusion for reality in contrast to spiritual sobriety. Prelest carries a connotation of allurement in the sense that the serpent beguiled Eve by means of the forbidden fruit. (Apart from its spiritual context, the word in Russian is often used in a positive sense of something charming, “lovely.”)
The site then quotes from St. Ignatius Brianchaninov and Archbishop Theophan of Poltava. First St. Ignatius:
Spiritual deception is the wounding of human nature by falsehood. Spiritual deception is the state of all men without exception, and it has been made possible by the fall of our original parents. All of us are subject to spiritual deception. Awareness of this fact is the greatest protection against it. Likewise, the greatest spiritual deception of all is to consider oneself free from it. We are all deceived, all deluded; we all find ourselves in a condition of falsehood; we all need to be liberated by the Truth. The Truth is our Lord Jesus Christ (John 8:32-14:6). Let us assimilate that Truth by faith in it; Let us cry out in prayer to this Truth, and it will draw us out of the abyss of demonic deception and self-delusion. Bitter is our state! It is that prison from which we beseech that our souls be led out ,that we may confess the name of the Lord(Ps. 141:8). It is that gloomy land into which our life has been cast by the enemy that hates and pursues us. It is that carnal-mindedness (Rom. 8:6) and knowledge Falsely so-called (I Tim. 6:20) wherewith the entire world is infected, refusing to acknowledge its illness, insisting, rather, that it is in the bloom of health. It is that “flesh and blood” which “cannot inherit the Kingdom of God”(I Cor. 15:50). It is that eternal death which is healed and destroyed by the Lord Jesus, Who is “the Resurrection and the Life” (John 11:25). Such is our state.
Archbishop Theophan expounds on this:
It is evident from these words of Isaac the Syrian that what we call prelest proper exists when a man starts trying to live above his capabilities. Without having cleansed himself of passions, he strives for a life of contemplation and dreams of the delights of spiritual grace. Thus the wrath of God befalls a man; because he thinks too highly of himself, God’s grace is withdrawn from him and he falls under the influence of the evil one who actively begins to tickle his vainglory with lofty contemplation and [spiritual] delights…
Briefly, the difference between “general prelest” and prelest in the particular sense of the word can, on the basis of the above. be expressed thus. General prelest is forgetting and not noticing one’s sinfulness. That which we call prelest proper is attributing to oneself righteousness when it does not actually exist. If a man thinks he is righteous, then his righteousness is not divine, but diabolical, foreign to the grace of God and to humility. One should recall the famous saying of Abba Poemen the Great: “I prefer a man who sins and repents to one who does not sin and does not repent. The first has good thoughts, for he admits that he is sinful. But the second has false, soul-destroying thoughts, for he imagines himself to be righteous” (Bp. Ignatius, Patericon, 75).