In algebra, the opposite of a ring is another ring with the same elements and addition operation, but with the multiplication performed in the reverse order.
More precisely, the opposite of a ring (R, +, ·) is the ring (R, +, ∗) whose multiplication ∗ is defined by a ∗ b = b · a. Ring addition is per definition commutative.
Properties
Two rings R1 and R2 are isomorphic if and only if their corresponding opposite rings are isomorphic. The opposite of the opposite of a ring is isomorphic to that ring. A ring and its opposite ring are anti-isomorphic.
A commutative ring is always equal to its opposite ring. A non-commutative ring may or may not be isomorphic to its opposite ring.
References
Opposite ring Wikipedia(Text) CC BY-SA