The requirement in trademark law that certain marks must obtain a level of market awareness or recognition from consumers prior to receiving any protection. Descriptive marks, as well as those that are primarily construed as surnames or as geographical indicators all require secondary meaning in order to be registered or to invoke court assistance stop infringers.
The type of market awareness required is recognition from in the minds of the relevant consuming public that the mark is used as a brand, and not merely used to describe a product, region, or someone's surname. For example, "McDonald" was once mostly thought of as a surname. When this name became widely used to sell burgers, then "McDonald's" has the power to bring to mind a restaurant chain in the minds of consumers. Once that happened, "McDonald's" acquired a second meaning, aside from its primary meaning as a surname, and as such, became capable of trademark protection.
Only "relevant consumers" are the ones who must have experienced the cognitive shift from viewing the mark as a descriptive term, surname or geographical term, to a brand-identifier. This includes not only purchasers, but also those who might someday be purchasers of the products bearing the mark in question.
Secondary meaning is generally acquired only after long-term exposure to the mark as a brand identifier, or else from extensive advertising. It can be proven by evidence such as a survey of relevant consumers, sales figures, trade literature referring to the mark, large advertising expenditures, or other proof of widespread advertising. Proof that a mark has been used exclusively for five years is prima facie evidence that the mark has acquired secondary meaning.