Having existed for a long time:
— A long-standing friendship.

synoniem: longstanding.

Roget 110: durable; lasting etc. v.; of long duration, of long-standing; permanent, endless, chronic, long-standing; intransient^†, intransitive; intransmutable^†, persistent; lifelong, livelong; longeval^†, long-lived, macrobiotic, diuturnal^†, evergreen, perennial; sempervirent^†, sempervirid^†; unrelenting, unintermitting^†, unremitting; perpetual etc. 112.    lingering, protracted, prolonged, spun out etc. v.. long-pending, long-winded; slow etc. 275.    ... meer laten zien