In telecommunication, the term user has the following meanings:

• A person, organization, or other entity (including a computer or computer system), that employs the services provided by a telecommunication system, or by an information processing system, for transfer of information.
  • A user functions as a source or final destination of user information, or both. Synonym subscriber.

• A person or process accessing an AIS by direct connections (e.g., via terminals) or indirect connections.

• • "Indirect connection" relates to persons who prepare input data or receive output that is not reviewed for content or classification by a responsible individual.

Source: from Federal Standard 1037C and from MIL-STD-188 and from the National Information Systems Security Glossary

In Wikimedia Projects (including wikipedia) you can access a user's page, pressing in the link that appears in the user's signature.

User is also a common term for someone who takes drugs, especially illegal ones.

When spoken with a particular affect in certain hacker contexts, the meaning is not unlike the immediately above use of user. See the jargon file definition of user.