Definition from Wiktionary, the free dictionary
- (set theory) Of a relation R on a set S, such that xRy if and only if yRx for all members x and y of S (that is, if the relation holds between any element and a second, it also holds between the second and the first).
- "Is a sibling of" is a symmetric relation.
- (cryptography) Using the same key (or keys that are trivially related) for both encryption and decryption.
Related terms 
symmetrical — see symmetrical