Dictionary.net

A trademark or service mark that describes the characteristics of the product or service to which it's attached. For instance, "Jiffy Lube" describes its purportedly fast service. Marks judged to be descriptive are initially considered legally weak and don't get much protection from the courts. If federal registration is sought, they are usually placed on the Supplemental rather than Principal Trademark Register, which also doesn't provide much protection. After a descriptive mark has been in use for five years, however, it can be moved to the Principal Register on the theory that it has by that time become well known through public exposure.