reciproka

The word matters because Esperanto keeps reflexive and reciprocal apart, and English does not. Ili amas sin says each of them loves himself; ili amas unu la alian says they love each other. When you need to be unmistakable, use unu la alian, and note that the -n lands on alian because that is the object. Reciproke as an adverb does the same job, and reciprokeco covers reciprocity in the abstract, which is where the word earns its keep in the movement's rather committee-flavoured prose.