Note: this page refers to dilution in the sense of trademark law. For the chemical concept of dilution, please refer to concentration.

Dilution is a concept and type of heightened protection in trademark law intended to protect the trademarks with the strongest identifiability from being used in noncompeting markets, thus "diluting" the singular power of the mark to identify one product. Some jurisdictions require additional registration of these trademarks as defensive marks in order to qualify for dilution protection.

Though trademark law is generally focused on the need for consumer protection, in many jurisdictions the concept of dilution has developed recently to protect trademarks as a property right, securing the investment the trademark owner has made in establishing and promoting a strong mark. The concept of dilution is much newer than the rest of trademark law; only in the mid-1990s did the United States enact a law against trademark dilution. A trademark is diluted when the use of similar or identical trademarks in other non-competing markets means that the trademark in and of itself will lose its capacity to signify a single source. In other words, unlike ordinary trademark law, dilution protection extends to trademark uses that do not confuse consumers regarding who has made a product. Instead, dilution protection law aims to protect sufficiently strong trademarks from losing their singular association in the public mind with a particular product, perhaps imagined if the trademark were to be encountered independently of any product (i.e., just the word Pepsi spoken, or on a billboard).

The strength a trademark must have to deserve dilution protection differs among jurisdictions, though it generally includes the requirement that it must be distinctive, famous, or even unique. Such trademarks would include instantly recognizeable brand names, such as Coca-Cola, or Sony, and unique terms that were invented (such as Exxon) rather than surnames (such as Ford) or ordinary words in language.

Another way of describing the necessary strength of a trademark may establish some basis for dilution protection from a consumer confusion standpoint. Truly famous trademarks are likely to be seen in many different contexts due to branching out or simple sponsorship, to the extent that there may be very few markets, if any, that a consumer would be surprised to see that famous trademark involved in. A prime example may be the past involvement of Coca-Cola in clothing lines.

Dilution is sometimes divided into two related concepts: blurring, or essentially basic dilution, which "blurs" a mark from association with only one product to signify other products in other markets (such as "Kodak shoes"); and tarnishment, which is the weakening of a mark through unsavory or unflattering associations. Not all dilution protection laws recognize tarnishment as an included concept.

Prior to specifically targeted laws being adopted, dilution protection was used in some jurisdictions to attack domain name infringement of trademarks (see Cybersquatting).