Masturbation is generally a taboo topic among religious Muslims and the very reason why it is so controversial is because most religious Muslims are against masturbation and forbid it. The general Arabic term for masturbation is known simply as "istimna" meaning putting out semen but many Muslim theologians use the term "Adah al-Sirriyyah"(the secret habit) and this very term shows how disliked masturbation is among conservative Muslims. Masturbation or Istimna is the stimulation of your own sexual organ to enjoy sexual pleasure. There are varying opinions among the four main schools of thought as mentioned here https://cjislam.weebly.com/why-masturbation-is-not-haram-10-convincing-proofs.html . Some completely forbid it while the others dislike it. When it comes to the textual sources of Islam which are the Quran and the authentic hadiths, the word masturbation is not mentioned. Islam is basically silent on masturbation and this clearly indicates that masturbation was never forbidden in Islam.