consulting is the activity or business of giving expert advice about a particular subject, and, used before a noun, describes someone who gives advice on a particular subject, as in a consulting lawyer or engineer.
The Cambridge Business English Dictionary treats the noun as uncountable and pairs it with fields such as financial, IT, and management consulting, and with nouns such as business, company, and firm. Dictionary.com defines the adjective as employed or involved in giving professional advice to the public or to those practicing the profession, as in a consulting physician, and gives a second sense: of, relating to, or used for consultation, as in a physician's consulting room. It records the word as formed from consult plus -ing, first recorded in 1790–1800.
The base verb describes the other side of the exchange. In the Oxford Learner's Dictionary, to consult is to go to somebody for information or advice, or to discuss something with somebody to get their permission or to help make a decision; Oxford glosses a consulting engineer as one who has expert knowledge and gives advice.
The neighbouring noun consultancy names either a company that gives expert advice on a particular subject to other companies or organizations, or the expert advice that a company or person is paid to provide.