The unoffical relationship of two people developed through sexting. Flirting, hooking up, and sexting a person but not being officially in a relationship. Similar to a fuck buddy but has emotional elements of a relationship as well even though neither party will admit it.
A: "So are you two in a relationship?"
B: "NOOOO! WE ARE NOT!"
A: "But you guys sext and hook up right?"
B: "Well yeah but it is definitely not a relationship"
A: "So its a sextlationship!"