steady relationship
From Longman Dictionary of Contemporary Englishsteady relationshipsteady relationshipRELATIONSHIPa serious and strong relationship that continues for a long time → steadyExamples from the Corpussteady relationship• But what of courses which do not have such a steady relationship?• Half are married or in steady relationships.• The highest earners have the most confidence in themselves, while those currently without a steady relationship are relatively lacking in confidence.