The phrase "rubber nipple" refers to a soft, flexible attachment that is made of rubber, usually used for babies to suck on while they drink from a bottle. Full definition
When first founded in 1920, the company focused on baby feeding products alone, including the first rubber nipple secured with a cap instead of being pulled over a bottle top. (bestcarseathub.com)
Parents can choose from the traditional rubber nipple, or nipples made from latex or silicone. (homeinstitute.com)
Its a flexible rubber nipple that you put on your own nipple to help baby latch on. (babycenter.com)