Symbiotic describes a relationship where two different living things — or two people, or two companies, or two countries — live closely together and each one needs what the other provides. The key is mutual need. Not one helping the other; both getting something, and both worse off if the other disappeared.

The word starts in biology, and that is still where it is most exact. Bees and flowering plants are symbiotic: the bee gets food, the flower gets carried pollen. A lichen on a rock is actually two organisms, a fungus and an alga, living so closely that people took them for one thing for centuries. Certain bacteria live in your gut and help you digest food while you feed and house them. In careful scientific use, symbiosis is the broad term for 'living together' and it has sub-types — mutualism (both gain), commensalism (one gains, the other is unaffected), even parasitism (one gains at the other's cost). But in normal use, including most science writing for the public, symbiotic just means the first one: both gain.

Outside biology it is one of the most-borrowed words in English. A magazine will call a director and her lead actor symbiotic; a business writer will describe a symbiotic relationship between a coffee shop and the bookstore next door; a music critic will say a bass player and a drummer have a symbiotic sense of time. All of these are fine and all of them are metaphor, drawing on the biological picture of two things joined so tightly that you can no longer treat them separately.

Though actually — listen to the tone, because it has drifted. Symbiotic sounds positive, so people reach for it whenever a relationship is close and productive. But there is a second, cooler use, especially in psychology and in political writing, where symbiotic means uncomfortably dependent: a symbiotic relationship between a mother and an adult child who cannot make a decision alone, or between politicians and the reporters who need them for stories. Same structure, mutual need, but the writer is warning you rather than admiring. You tell which one is meant from the sentence around it, not from the word.