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The term defining set is used when describing fish patterns.
A fish pattern has 2 interacting sets of constraints. These are usually rows or columns, but modern fish variations also use boxes. The 2 sets are equal in size. One of the sets is the defining set and the other is the secondary set.
In a fish pattern, the defining set of constraints has candidates for the selected digit which are located only within the constraints of the secondary set. The secondary set has surplus candidates which are not located inside the defining set. These surplus candidates can be eliminated.
So for example in an X-wing the above would equate to this: In an X-wing pattern, the defining set (rows) has candidates for the selected digit which are located only on the columns of the secondary set. The columns have surplus candidates and these can be eliminated.
In forum discussions the defining set is often called base set.