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  1. (MATH) Another name for an annulus.

  2. (MATH) A set (see set theory) with the following properties: it has two binary operations, addition and multiplication (+ and ×); it is an Abelian group under addition; multiplication is associative, and distributive over addition; and a and b are members of the set, a×b has a unique value c, also a member of the set. If there is a member n of the set such that a × n = a for every member a of the set, the ring is a ring with unity. A ring in which multiplication is commutative is a commutative ring.

    Compare with field.

       • ALGEBRA

  3. (ASTRON) See planetary ring.