Definition:
A best of breed system is the best system in its referenced niche or category. Although it performs specialized functions better than an integrated system, this type of system is limited by its specialty area.
To fulfill varying requirements, organizations often use best of breed systems from separate vendors. However, maintaining multiple systems provides little cross connectivity, which creates maintenance and integration challenges.
Further Reading:
Book: Playing to Win by A G Lafley and Rogel L. Martin