The act of deliberately inflicting harm on ones own body without suicidal intent or plans of bodily modification (eg. piercing your ears isn't self-injury). Typically self-injury is practiced to block mental pain with physical pain, or to provide the self-injurer with something that they can feel if they are emotionally numb. Common forms of self-injury include cutting with a sharp object such as a razor or a knife, burning (with an open flame, chemicals, a heated object (technically, this is branding, not burning), or friction from rubbing with an object such as a pencil erasor), scratching or scraping, and hitting (whether with ones own hands or another object).
Although it is sometimes misinterpreted as a suicide attempt, self-injury is actually a coping mechanism used to deal with powerful negative feelings and prevent suicide.