Reciprocal describes something that's the same on both sides. If you and your sister are in a big fight on a long car trip, you might resolve it through a reciprocal agreement that you'll stop poking her and she'll stop reading road signs out loud.
The word mutual is a near synonym in most uses: reciprocal/mutual friendship, describing, a relationship in which two people feel the same way about each other, or do or give similar things to each other. If you tell someone you like them and they say, "The feelings are reciprocal," that means they like you too. In math, a reciprocal is a number that when multiplied by a given number gives one as a product.
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