Since the Christian tradition emerged from Judaism, it is advisable to read the relevant articles on this topic from that tradition. The Bible is nonetheless quite clear about masturbation, although the word itself does not appear in the scripture. Consider these verses:
And if a man has an emission of semen, he shall bathe his whole body in water, and be unclean until the evening. And every garment and every skin on which the semen comes shall be washed with water, and be unclean until the evening. - Leviticus 15
Flee Fornication. Every sin that a man doeth is without the body; but he that committeth Fornication sinneth against his own body. What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's. - 1 Corinthians 6
Masturbation uses imagery and imagination to stimulate sexual desire. In the Bible, the use of the eyes is clearly related to sexuality.
I made a covenant with my eyes, and why should I gaze upon a woman? [...] If my heart was enticed to a woman... that is lewdness, and it is iniquity deserving punishment. For it is fire; it consumes to destruction... - Job 31
...do not let sin reign in your mortal body so that you obey its evil desires. Do not offer the parts of your body to sin, as instruments of wickedness, but rather offer yourselves to God, as those who have been brought from death to life; and offer the parts of your body to him as instruments of righteousness. For sin shall not be your master for you are not under law but under grace. - Romans 6:11-14
For this is the will of God, even your sanctification, that ye should abstain from Fornication: That every one of you should know how to possess his vessel in sanctification and honour; Not in the lust of concupiscence, even as the Gentiles which know not God... [...] For God hath not called us unto uncleanness, but unto holiness. - 1 Thessalonians 4
A Comment by C. S. Lewis
For me the real evil of masturbation would be that it takes an appetite which, in lawful use, leads the individual out of himself to complete (and correct) his own personality in that of another (and finally in children and even grandchildren) and turns it back; sends the man back into the prison of himself, there to keep a harem of imaginary brides. And this harem, once admitted, works against his ever getting out and really uniting with a real woman. For the harem is always accessible, always subservient, calls for no sacrifices or adjustments, and can be endowed with erotic and psychological attractions which no woman can rival. Among those shadowy brides he is always adored, always the perfect lover; no demand is made on his unselfishness, no mortification ever imposed on his vanity. In the end, they become merely the medium through which he increasingly adores himself. Do read Charles Williams' Descent into Hell and study the character of Mr. Wentworth.
And it not only the faculty of love which is thus sterilized, forced back on itself, but also the faculty of imagination. The true exercise of imagination in my view, is (a) To help us understand other people, (b) To respond to, and, some of us, to produce art. But is has also a bad use: to provide for us, in shadowy form, a substitute for virtues, successes, distinctions, etc. which ought to be sought outside in the real world--e.g., picturing all I'd do if I were rich instead of earning and saving. Masturbation involves this abuse of imagination in erotic matters (which I think bad in itself) and thereby encourages a similar abuse of it in all spheres. After all, almost the main work of life is to come out of our selves, out of the little dark prison we are all born in. Masturbation is to be avoided as all things are to be avoided which retard this process. The danger is that of coming to love the prison. - From a letter to a Mr. Masson dated March 6, 1956 in the Wade Collection at Wheaton College in Wheaton, IL
Incubi & Succubi
Masturbation utilizes creative sexual energy, but directed towards images projected in the mind. This energy creates mental forms, often called incubi (masculine forms) or succubi (feminine forms).
Lilith grows out of the wind just as the salamander grows from the fire. Lilith represents the classical example of the succubus in Jewish mythology. The incubus is a spirit which, taking the semblance of a man, has intercourse with mortal women. The succubus is a similar spirit which in the form of a woman behaves in a like manner with mortal men. The Hebrew Lilith was regarded as queen of the succubi by the theologians who spent much time investigating such matters. - Solomon ben Abraham (Salerno, 1160 CE)
St. Augustine stated that “the devils (of Lilith) do indeed collect human semen, by means of which they are able to produce bodily effects.”
As for the unclean spirits, there are males among them and there are females. The males are they which unite with the souls which inhabit a female form, but the females are they which are mingled with those in a male form, through one who was disobedient. And none shall be able to escape them, since they detain him if he does not receive a male power or a female power - the bridegroom and the bride.One receives them from the mirrored bridal chamber. When the wanton women see a male sitting alone, they leap down on him and play with him and defile him. So also the lecherous men, when they see a beautiful woman sitting alone, they persuade her and comple her, wishing to defile her. But if they see the man and his wife sitting beside one another, the female cannot come into the man, nor can the male come into a woman. - The Gospel of Philip