The USPTO will refuse to register a trademark for certain reasons.
Several common grounds for refusal include:
The proposed trademark is merely descriptive or deceptively misdescriptive of the applicant's goods or services.
Examples of merely descriptive trademarks which could be refused include: "medical guide" for website services featuring medical guides, "denim" for jeans, and "spicy sauce" for salsa.
The proposed trademark consists of or comprises immoral, deceptive, or scandalous matter.
The proposed trademark may disparage or falsely suggest a connection with persons (living or dead), institutions, beliefs, or national symbols, or bring them into contempt or disrepute.
The proposed trademark consists of or comprises the flag, coat of arms, or other government insignia of the United States or any state, municipality, or foreign nation.
The proposed trademark consists of or comprises a name, portrait or signature identifying a particular living individual, except by that individual's written consent; or the name, signature, or portrait of a deceased U.S. President during the life of his widow, if any, except by the widow's written consent.
The proposed trademark resembles a mark already registered with the USPTO, so that use of the applicant's trademark would likely cause confusion, mistake or deception.
The proposed trademark is primarily geographically descriptive or deceptively geographically misdescriptive of an applicant's goods or services.
The proposed trademark is primarily merely a surname.
Matter that, as a whole, is functional. For example, the word "Fragile" cannot be used to trademark labels bearing that word to connote fragility of a package's contents.