Pacifier: An artificial nipple, usually made of plastic, upon which an infant can suck to gain some solace and quiet down. A pacifier is called by other names in other countries including a dummy in the U.K. Pacifiers have been claimed to cause early weaning. However, a study (JAMA, July 18, 2001) was done of women who were planned to breastfeed for at least 3 months.