A rainmaker, in a business context, is an employee who generates an unusual amount of new business or revenue for a company.
The new business or revenue that a rainmaker brings in is often from an unknown or overlooked source, creating the impression that the assets were created from nothing or through mysterious means. In fact, the rainmaker's success is typically the predictable effect of being an outgoing, motivated and energetic employee with a large social social network of contacts.
Rainmaker is also used in a broader sense to refer to any prolific businessperson. For example, an entrepreneur who develops a lot of new companies might be called a rainmaker.
The term is a reference to rainmakers of the old west who were contracted to make it rain in drought-stricken areas by performing rain dances and other ceremonial activities.