Definitions for safe sex
Random House Webster's College Dictionary
sexual activity in which precautions are taken to prevent diseases transmitted by sexual contact.
Origin of safe sex:
sexual activity (especially sexual intercourse) with the use of measures (such as latex condoms) to avoid the transmission of disease (especially AIDS)
Sexual activity engaged in by people who have taken precautions to protect themselves against sexually transmitted diseases (STDs).
U.S. National Library of Medicine
Sexual behavior that prevents or reduces the spread of SEXUALLY TRANSMITTED DISEASES or PREGNANCY.