The term 'dater' refers to the word 'dating' or 'to date someone'. The male and female (or male and male or female and female, whatever floats your boat) who date one another are known as the daters and under no circumstances relates to the couple being girlfriend and boyfriend (or boyfriend and boyfriend or girlfriend and girlfriend, again whatever floats your boat). However, if the couple refer to one another as being their dater, there is a strong possibility that this will lead to the daters becoming girlfriend and boyfriend (or boyfriend and boyfriend or girlfriend and girlfriend, yet again whatever floats your boat). This is so, as referring to one another as their 'dater' signifies a strong bond between the two people
Male: hey dater, how r u?
Female: hey dater, i am good thanks
A person you are dating, but not offically boyfriend/girlfriend with yet.
"Hi! Who's this?"
"Hey, this is my...dater"
"Hey, this is my...gir-DATER"