A core competency is a concept in management theory introduced by C. K. Prahalad and Gary Hamel. It can be defined as "a harmonized combination of multiple resources and skills that distinguish a firm in the marketplace" and therefore are the foundation of companies' competitiveness. 
Core competencies fulfill three criteria:
Provides potential access to a wide variety of markets.
Should make a significant contribution to the perceived customer benefits of the end product.
Difficult to imitate by competitors.
For example, a company's core competencies may include precision mechanics, fine optics, and micro-electronics. These help it build cameras, but may also be useful in making other products that require these competencies.
Core Competence A core competency results from a specific set of skills or production techniques that deliver additional value to the customer. These enable an organization to access a wide variety of markets.