An argument that has existed for a long time between two people or groups, causing a lot of anger or violence. (Cambridge Dictionary)
A prolonged and bitter quarrel or dispute. (Oxford Dictionaries)
A state of prolonged mutual hostility, typically between two families or communities, characterised by murderous assaults in revenge for previous injuries. (Oxford Dictionaries)
A mutual enmity or quarrel that is often prolonged or inveterate. (Merriam – Webster)
A feud is a quarrel in which two people or groups remain angry with each other for a long time, although they are not always fighting or arguing. (Collins Dictionary)
Vin Diesel And The Rock End Their Feud, Both Will Star In Fast 9.
Freud versus Jung: a bitter feud over the meaning of sex.