Various Definition of Customer Service

Customer service (also known as Client Service) is the provision of service to customers before, during and after a purchase. According to Turban et al, 2002, “Customer service is a series of activities designed to enhance the level of customer satisfaction – that is, the feeling that a product or service has met the customer expectation.”

Its importance varies by product, industry and customer. As an example, an expert customer might require less pre-purchase service (i.e., advice) than a novice. In many cases, customer service is more important if the purchase relates to a “service” as opposed to a “product".
Customer service may be provided by a person (e.g., sales and service representative), or by automated means called self-service. Examples of self service are Internet sites.
Customer service is normally an integral part of a company’s customer value proposition.

Source: Wikipedia (http://en.wikipedia.org/wiki/Customer_service)

Customer service is an organization's ability to supply their customers' wants and needs.
Customers and business managers alike like to talk about what good customer service is (and isn't), but I think this definition by ACA Group sums up what excellent customer service is beautifully: "excellent customer service (is) the ability of an organization to constantly and consistently exceed the customer's expectations."
Accepting this definition means expanding our thinking about customer service; if we're going to consistently exceed customers' expectations, we have to recognize that every aspect of our business has an impact on customer service, not just those aspects of our business that involve face-to-face customer contact.
Improving customer service involves making a commitment to learning what our customers' needs and wants are, and developing action plans that implement customer friendly processes.

Source: CSM Website (http://www.customerservicemanager.com/definition-of-customer-service.htm)

Customer service is often seen as an activity, performance measurement and a philosophy. That is why, generally speaking, one single definition of customer service does not exist..

Here are some definitions of customer service in use today:

"Customer service is the ability to provide a service or product in the way that it has been promised"

"Customer service is about treating others as you would like to be treated yourself"

"Customer service is an organization's ability to supply their customers' wants and needs"

"Customer Service is a phrase that is used to describe the process of taking care of our customers in a positive manner"

"Customer Service is any contact between a customer and a company, that causes a negative or positive perception by a customer"

"Customer service is a process for providing competitive advantage and adding benefits in order to maximize the total value to the customer"

"Customer Service is the commitment to providing value added services to external and internal customers, including attitude knowledge, technical support and quality of service in a timely manner"

"Customer service is a proactive attitude that can be summed up as: I care and I can do."

Source: About.Com (http://sbinfocanada.about.com/cs/marketing/g/custserv.htm)