A pretender is one who maintains a claim that he is entitled to a position of honour or rank, which may be occupied by an incumbent (usually more recognised), or whose powers may currently be exercised by another person or authority. Most often, it refers to a former monarch, or descendant thereof, whose throne is occupied or claimed by a rival or has been abolished.
The term "claimant" is sometimes preferred, but the term "pretend" in itself is not pejorative in this context. The original meaning of the English word comes from the French word prétendre (and before that, the Latin praetendo meaning "to stretch out before"), and originally meant "to put forward, to profess or claim". A pretender was, therefore, simply one who put forward or professed a claim to a title or, in modern terms, a claimant. Only later did the word acquire its modern sense of professing or claiming falsely.
The term "pretender" applies to claimants with arguably genuine rights (as the various pretenders of the Wars of the Roses who regarded the de facto monarch as an usurper). It can also be used for those possessing an arguable right to a position who do not actively claim it, as well as impostors with wholly fabricated claims (as pretenders to Henry VII's throne Lambert Simnel and Perkin Warbeck attest). People in the latter category often assume the identities of deceased or missing royalty to support their claim, and are sometimes referred to for clarity as false pretenders or royal impersonators. A Papal pretender is called an Antipope.
Ancient Rome knew many pretenders to the offices making up the title of Roman Emperor, especially during the crisis of the Third Century.