mutual

1470–80; < Middle French mutuel < Latin mūtu(us) mutual, reciprocal ( mūt(āre) to change (see mutate) + -uus deverbal adj. suffix) + Middle French -el (< Latin -ālis) -al¹

1. Mutual, reciprocal agree in the idea of an exchange or balance between two or more persons or groups. Mutual indicates an exchange of a feeling, obligation, etc., between two or more people, or an interchange of some kind between persons or things: mutual esteem; in mutual agreement. Reciprocal indicates a relation in which one act, thing, feeling, etc., balances or is given in return for another: reciprocal promises or favors.

The earliest (15th century) and still a current meaning of mutual is “reciprocal,” specifying the relation of two or more persons or things to each other: Their admiration is mutual. Teachers and students sometimes suffer from a mutual misunderstanding. Mutual soon developed the sense of “having in common, shared”: Their mutual objective is peace. This latter sense has been in use since the 16th century and is entirely standard. It is occasionally criticized, not on the grounds of ambiguity but on the grounds that the later sense development is somehow wrong. Mutual in the sense of “shared” may have been encouraged by the title of Charles Dickens's novel Our Mutual Friend (1864–65), but Dickens was not the innovator. The fact that common also has the sense “ordinary, unexceptional” and “coarse, vulgar” may have contributed to the use of mutual instead of common in designating a shared friend.