Autoeroticism is the practice of stimulating oneself sexually. The term was popularise toward the end of the 19th century by British sexologist Havelock Ellis, who defined autoeroticism as "the phenomena of spontaneous sexual emotion generated in the absence of an external stimulus proceeding, directly or indirectly, from another person." The most common autoerotic practice is masturbation, and the two terms are often used as synonyms, although masturbation can also occur with a partner.