consult

A consultant (from Latin: consultare "to deliberate") is a professional (also known as expert, specialist, see variations of meaning below) who provides advice or services in an area of specialization (generally to medium or large-size corporations). Consulting services generally fall under the domain of professional services, as contingent work.
The Harvard Business School defines a consultant as someone who advises on "how to modify, proceed in, or streamline a given process within a specialized field".

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