client

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The party for which professional services are rendered, as by an attorney.
• noun A customer or patron.
• noun A person using the services of a social services agency.
• noun One that depends on the protection of another.
• noun A client state.
• noun Computers A computer or program that can download files for manipulation, run applications, or request application-based services from a file server.

from The Century Dictionary.

• noun In Roman antiquity, a person who was under the guardianship and protection of another of superior rank and influence, called his patron.
• noun In a general sense, one who lives under the patronage of, or whose interests are represented by, another.
• noun In the middle ages, any follower of a noble or knight; an inferior soldier, mounted or on foot; a vassal.
• noun One who puts a particular interest into the care and management of another; specifically, one who applies to a lawyer for advice and direction in a question of law, or commits his cause or his legal interests in general to a lawyer's management.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Rom. Antiq.) A citizen who put himself under the protection of a man of distinction and influence, who was called his patron.
• noun A dependent; one under the protection of another.
• noun (Law) One who consults a legal adviser, or submits his cause to his management.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A customer, a buyer or receiver of goods or services.
• noun computing The role of a computer application or system that requests and/or consumes the services provided by another having the role of server.
• noun Person who receives help or advice from a professional person (ex. a lawyer, an accountant, a social worker, a psychiatrist, etc).
• noun law A person who employs or retains an attorney to represent him or her in any legal matter, or one who merely divulges confidential matters to an attorney while pursuing professional assistance without subsequently retaining the attorney.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who pays for goods or services
• noun a person who seeks the advice of a lawyer
• noun (computer science) any computer that is hooked up to a computer network