Of long duration or existence: a long-standing friendship. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "long-standing" in a sentence
"Obama encouraged the government of Bahrain, which he called a "long-standing partner," to lead a "meaningful, inclusive, non-sectarian and responsive" dialogue with its people."
"Debby formally sought therapy during and after college for what she called long-standing “eating disorders.”"
"Rawlings and Sivia, 49, were described as long-standing friends, with police saying the pair had known each other since they were teenagers."