The phrase "longstanding collaboration" means a partnership or working relationship that has been going on for a long time. It implies that the collaboration has been established, strong, and enduring over a significant period. Full definition
The program was born from longstanding collaboration between Dash, now the top payments focused digital currency and BlockCypher, the leading Blockchain Web Services (BWS) ™ provider. (the-blockchain.com)
The research builds on longstanding collaborations involving laboratories around the world, and scientists in the fields of chemistry, material science, physics and geochemistry. (sciencedaily.com)
We also have longstanding collaborations with GlaxoSmithKline to study inhibitors of the MAPK pathway in immune cells and melanoma, tumour antigens and predictive immune gene signatures. (onjcancercentre.org)