Complex sentence: Market Definition is a tool used by the Commission to enable it to define markets in which firms may operate, which as a result allows the Commission to identify market shares and consequentially whether that firm is in a dominant position within a relevant market.
Split into simpler sentences: Market Definition is a tool used to enable the Commission to define markets in which firms may operate. As a result, market definition allows the Commission to identify market shares and consequentially whether that firm is in a dominant position within a relevant market.