Is masturbation sin in Christianity ?

No its not a sin. It would be concerning if it became an obsession and kept from other healthy things you do in your life.

The Bible does not specifically mention masturbation, the deliberate self-stimulation that produces sexual arousal and orgasm.

Sexual offenses such as premarital sex (fornication), homosexuality, adultery, and bestiality are plainly condemned as gross sins in the Bible, masturbation is not mentioned.

Masturbation was common in the Greek-speaking world during Bible times, and several Greek words were used to describe the practice.

Interestingly, not one of these words is used in the Bible.

Masturbation is not a form of gross sexual immorality, such as fornication. Jude 7

Anything that feels good is a 'sin' in Christianity.

Sexual intercourse in the missionary position for the express intent of reproduction the only thing that is allowed.

It must not be an issue being that it is not mentioned in scripture.  What you think about while doing it, could be a sin though. Rock on!

Traditional Catholic

Encouraging lust and mentalities driven by lust is the 'sin' because those mentalities are part of the processes of early or unnatural death among ourselves or others, (the definition of 'sin,' James 1:15).

The Church discourages auto-erotic activities for this reason. However, the Holy Spirit replaces this experience with something better and more satisfying using dreams and visions. They include numerous types of ecstasy, including sexual ecstasy as it was intended to be experienced, rather than aroused by fleshly lust.

It is not 'getting off' that is the sin. It is what we use to arouse ourselves and the mentalities it creates.

If an arriving dream or vision arouses you without your 'help' and is based on the following 'godly' things, it is perfectly healthy, good and blessed by GOD. Sexual arousal should come from the following mentalities:

8"Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, dwell on these things." (Philippians 4:8). 

These contemplations and intimate experiences are a form of 'prayer' and communication which are supposed to guide us to others sharing in them with us, including to our mate, (if being married is GOD's will for us), or to give us a deeper mystical appreciation of the relationship between Christ and His 'Bride' (Ephesians 5:25-32), if marriage is not GOD's will for us.

By contrast, fleshly or earthly 'lust' manifests in predictable ways. The 'essentials' (ordinances inspired by the Holy Spirit and delivered to us by the Apostles, Acts 15:28-29, 16:4), later became the basis of all Church canon and ecclesiastical law. They describe for us the forms of 'arousal' which should now repulse us when we abide in the Holy Spirit. Briefly, they include: 

Arousal from contact with raw blood or violent images, lifestyles, etc...; arousal from strangulation, (which includes drowning, poisoning, etc...), and from the worship of 'celebrity' or its idols. This form of sexuality is known as 'porneia,' usually translated 'fornication.'

Dreams and visions from GOD will NOT contain these images, thoughts, or forms of arousal. Any other form of arousal should be welcomed, including anything you feel compelled to do in response. They are not 'sin' and the mentalities involved will not contribute to anyone's needless death when you apply them to your life and personal choices. They are also not always 'sexual' in nature, since GOD provides many even more powerful forms of ecstasy. They have inspired some of the world's greatest art, music, drama and social movements. 
 

Self-abuse is a gateway to tentacle porn, abortions and communism. 

It's not the masturbation in and of itself, it's the porn and images required to commit the act that is sinful. 

The Bible speaks in the Old testament and the New testament of what to do if a man shall have an emission he is to wash himself and any cloth that the omission touches and it be considered ceremonially unclean until the sun goes down. the New testament speaks about the sin of a man which does not lead unto death and if someone should see him do it should reprove him gently. The Bible says that we are to put away childish behaviors and to grow up and to become men of God which includes giving up child as masturbation which is the sin which does not lead to death but it's not something we're supposed to do the rest of our life it's something that children do well they're growing up to be men when you're a man you're expected to get married and have children and your sex comes from your wife. And the woman's sex comes from the man. You outgrow your child is behaviors and you gay in the behaviors of a man or a woman of God.masturbation is considered an uncleanness a sin that does not lead to death something that you're supposed to outgrow. And that's the way it should be treated.if you're a grown man or woman of marriage age you're supposed to give up masturbation and find a lawful wife or a lawful husband and take care of each other's needs

According to the Catechism of the Catholic Church: 2352 By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action."138 "The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose." For here sexual pleasure is sought outside of "the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved."139

To form an equitable judgment about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors that lessen, if not even reduce to a minimum, moral culpability.

IF YOU CANNOT HELP IT, THEN IT IS ONLY A VENIAL SIN, BUT IF YOU DONT STOP IT USING THE GRACE GOD GIVES YOU TO DO SO, IT TURNS INTO MORTAL SINS BECAUSE OF THE HOLDING ON TO THE IMPURE ACTS THAT YOU THINK ABOUT WHEN DOING IT. THE 6TH COMMANDMENT, THOU SHALL NOT COMMIT ADULTERY, COVERS ALL IMPURE ACTS NOT JUST ADULTERY, BUT PLEASE READ THIS>>>>

YOU MAY NOT KNOW THIS, BUT YOU ARE PLEASING GOD VERY MUCH HERE, WHY? BECAUSE YOU WANT TO BE GOOD JUST FOR HIM. WHEN ANYONE ATTEMPTS A REAL DESIRE AT BEING GOOD THE EVIL ONE WORKS HARDER THAN EVER TO PERSUADE YOU TO FORGET IT, AT FIRST THE DESIRES TO DO BAD SEEMS REALLY STRONG, BUT GOD IS ON YOUR SIDE. DONT EVER THINK ABOUT IT IF YOU CAN HELP IT. REMEMBER JESUS WHEN HE WAS CARRYING HIS CROSS, HE FELL THREE TIMES, BUT GOT RIGHT BACK UP IN GREAT PAIN AND MOVED FORWARD JUST FOR LOVE OF YOU AND I. HE WAS SHOWING US AND EXAMPLE THAT IF WE SIN, GET RIGHT BACK UP AND RETURN TO HIM IMMEDIATELY FOR FORGIVENESS AND WHEN YOU ARE SINCERE, HE WILL FORGIVE YOU AS MANY TIMES AS IT TAKES TO CONQUER YOUR SINS. THERE ARE NO AMOUNT OF SINS GREATER THAN THE LORDS MERCY. BUT YOU MUST RETURN TO HIM ASAP IF YOU DONT SOON OR LATER YOU WILL NOT COME BACK TO HIM AT ALL. GOD KNOWS WHEN YOU ARE REALLY TRYING OR NOT AND SO DO YOU! YOU MAY GET THOUGHTS LIKE I AM A HYPOCRITE AT FIRST BUT PAY NO ATTENTION TO THAT EITHER, IT IS FROM THE EVIL ONE. JESUS SAID "THOUGH YOUR SINS BE AS SCARLET, I SHALL MAKE THEM WHITE AS SNOW"
JUST BE HONEST WITH GOD AND BEFORE YOU KNOW IT YOU WILL BE FREE FROM SINS, BUT THE MAIN STRENGTH FROM SIN COMES FROM HOLY COMMUNION, AND SECRET PRAYER AS MUCH AS POSSIBLE. YOU WILL NOT ONLY GROW IN STRENGTH WITH SINS BUT YOU WILL TURN AND LOVE THE LORD YOUR GOD. IF YOU CAN GO TO THE PRIVACY OF YOUR BEDROOM AND KNEEL BEFORE HIM AND TELL HIM YOU WANT TO START FRESH AND NEED HIS HELP. HE WILL POUR ON HIS GRACE OUT OF LOVE FOR YOU AND MAKE YOU STRONGER THAN YOU EVER THOUGHT POSSIBLE, THIS IS NOT DONE OVER NIGHT, BE PATIENT AND TRUST THE LORD TO PULL YOU OUT OF YOUR SLUMP.
I WILL OFFER UP THE HOLY SACRIFICE OF THE MASS FROM NOW ON, JUST FOR YOU.
NOTE ALWAYS REMEMBER A SEXUAL THOUGHT OR THOUGHTS IS NOT SINNING, A THOUGHT IS NEVER A SIN, ITS WHEN YOU EMBRACE IT AND GIVE INTO YOUR URGES THAT IS THE SIN, BUT JESUS KNOWS OUR WEAKNESS'S BETTER THAN WE DO, JUST DO AS RECOMMENDED AND BEFORE YOU KNOW IT, YOU WILL BE IN THE LORDS SECRET ARMY OF HIS FAITHFUL. AMONG TRUE CHRISTIANS WHO WANT ONLY TO PLEASE GOD OUR FATHER.

REMEMBER WHAT JESUS SAID, IF YOU LET YOURSELF LUST AFTER A PERSON IN YOUR MIND, YOU HAVE ALREADY COMMITTED THE SIN.