(1) In VSAM, a field, other than the primary key, of fixed length and position in a record. A set of alternate keys is used to build an alternate index that provides an alternative or secondary path for access to the data set. There can be any number of alternate keys in a record and they need not be unique.
(2) In z/OS Virtual Storage Access Method (VSAM), one or more bytes within a data record used to identify the data record or control its use. Unlike the prime key, the alternate key can identify more than one data record. It is used to build an alternate index or to locate one or more base data records using an alternate index. See also key field, alternate-index record.