A sadist is a person who is aroused by the suffering or humiliation of another person. Sadism with a consenting partner is not always illegal. Laws vary depending upon where you live.
Sadism involves causing physical or psychological pain or suffering to another person.
As long as it occurs with a consenting partner, sexual sadism is not considered to be a psychological disorder. It is considered a disorder when it causes unhappiness to the person with it or causes problems with work, social setting, or family. If the other person is not willing, sadism can be a severe and even criminal disorder. Like some masochists, some sadists require the pain or humiliation in order to function sexually. Others may engage in more typical sexual activities at some times and sadistic activities at other times.