Symbiosis
Etymology
From Greek symbiōsis, "living together" (syn-, "together," plus bios, "life"). The German botanist Heinrich Anton de Bary gave it its biological meaning in 1879, defining it broadly as the living together of unlike organisms — pointedly including harmful pairings, not just cozy ones. Popular use has narrowed it toward mutual benefit, but the science keeps the wider sense.
noun · /sim-bee-OH-sis/
A close, long-term living relationship between different organisms — which may benefit both, only one, or one at the other’s expense.
What it names
Any lasting arrangement where two very different parties are tightly bound together, for better or worse, so that each shapes the other’s fate.
Example
The old café and the bookshop next door lived in a comfortable symbiosis, each sending its customers to the other.
Related idea
Mutualism is the specific case where both sides gain; parasitism is the case where one gains at the other’s cost. Symbiosis is the umbrella over all of them.