A trademark is a word, or some other perceptible symbol, that identifies to the public the source of a particular product or service, and distinguishes that source’s product from those of others in the market. 

For example, the word “Quest” identifies to the public a particular provider of medical diagnostic services, and distinguishes Quest’s services from its competitors in the market for such services, like LabCorp or BioReference Laboratories. Likewise, the name “Rolaids” identifies the source of an antacid medication, and distinguishes this product from essentially identical products in the market, e.g.  “Tums”.   

Most word trademarks are proper adjectives that modify nouns: Rolaids antacid medication; Tums antacid medication. The marketing departments of companies that own these trademarks promote their use as proper adjectives rather than nouns. This is because if a brand like Rolaids saturates the market to the extent the public associates the brand name with a product rather than with a particular source of it, the trademark has become generic: instead of “I need to take an antacid” people are more likely to say “I need to take a Tums”.

It is paradoxical: the more the public associates a trademark with a product, the greater the chance it may be deemed generic, and no longer protectable. Because Kleenex brand facial tissue became so popular, we now refer to all facial tissue using the noun “kleenex” regardless of its source, which weakens the extent Kimberly-Clark, owner of the Kleenex trademark, can prevent others from using it.