The Free On-line Dictionary of Computing (30 December 2018):

direct mapped cache

    A cache where the cache location for a given
   address is determined from the middle address bits.  If the
   cache line size is 2^n then the bottom n address bits
   correspond to an offset within a cache entry.  If the cache
   can hold 2^m entries then the next m address bits give the
   cache location.  The remaining top address bits are stored as
   a "tag" along with the entry.

   In this scheme, there is no choice of which block to flush on
   a cache miss since there is only one place for any block to
   go.  This simple scheme has the disadvantage that if the
   program alternately accesses different addresses which map to
   the same cache location then it will suffer a cache miss on
   every access to these locations.  This kind of cache
   conflict is quite likely on a multi-processor.  See also
   fully associative cache, set associative cache.