A person whose influence can initiate progress or ensure success
(Source: Merriam-Webster)

An employee who creates a large amount of unexpected business or consistently brings in money in critical times
An executive with exceptional ability to attract clients, use political connections or increase profits, etc.
(Source: Wiktionary)

A person who brings in new business and wins new accounts almost by magic ... generates substantial new business or additional cash flow from sources sometimes outside established business channels, and sometimes by connecting with people in non-traditional or hidden markets
(Source: Wikipedia)