What’s the difference between a customer and a client? Substantially, not much — but as we all know on some level, the exchange of currency for goods and services is more about the style than the substance. Savvy merchants have blurred the distinction in the interests of encouraging business by conferring prestige on potential purchasers.
First, word origins: Customer’s root word, custom, ultimately derives from the Latin verb consuescere, “to accustom,” and the sense of a person who buys something from another perhaps stems from the idea of purchasing as being a habit. Client (the plural can be clients or clientele) also comes from Latin, in the form of clientem, “follower,” which may be related to the root word of incline. This sense persists in the phrase “client state,” referring to a nation dependent on another for security or other support.
The two terms have traditionally differed widely in usage: A customer is simply a recipient of products or services in exchange for money. Even though the relationship to the provider might be long lasting, the sense is of discrete exchanges. By contrast, a client is engaged in a more qualitative relationship in which the provider generally applies professional skills to offer often intangible commodities such as legal services, insurance policies, and the like. (Another distinction is that a customer is more likely to visit a retail establishment, whereas a client may more easily receive services without being physically present at the place of business. The escalation of mail-order business spurred by online retailing, however, has blurred this distinction.)
Because of the greater perceived value associated with provision of professional services, businesses not normally classified as providers of such have taken to referring to their customers as clients. Technically, there’s nothing wrong with that; why shouldn’t an auto mechanic refer to people with car trouble as clients rather than customers? As I mentioned above, it’s all about the prestige: A streetwalker services customers, but an escort sees clients.
Synonyms for customer and client are available, but they have their limits: A buyer is someone who pays for something, but the word also refers to someone employed by or otherwise associated with a company who purchases things wholesale to later be sold as is or as part of a retail product by that company.
Patron is more limited in connotation than customer or client; it generally refers to someone purchasing an aesthetic experience such as a performance or a meal rather than carting items from a shelf to a checkout stand. It also applies, however, to a supporter (as in “a patron of the arts”) or a guardian (as in “a patron saint”).
Guest is an elegant way to describe someone acquiring lodging or otherwise remaining on the business premises for an extended time but seems pretentious for other usages. Meanwhile, consumer seems too impersonal and is best reserved for referring, in singular or plural form, to connote a typical person who buys products or services or the general public in that role.
Other synonyms such as user (or “end user”) are highly specific to technological products and services, and yet others are clunky (purchaser, vendee) or describe someone on the way to becoming a customer or client but not yet there (prospect, shopper). Your best bet is to weigh customer and client and, with candor, determine which is more appropriate for the context.