A capitonym is a word that changes its meaning (and sometimes pronunciation) when it is capitalized; the capitalization usually applies due to one form being a proper noun or eponym. It is a portmanteau of the word capital with the suffix -onym. A capitonym is a form of homograph and – when the two forms are pronounced differently – also of heteronym. In situations where both words should be capitalized (such as the beginning of a sentence), there will be nothing to distinguish between them except the context in which they are used.
Although some pairs, such as march and March, are completely unrelated, in other cases, such as august and catholic, the capitalized form is a name that is etymologically related to the uncapitalized form. For example, August derives from the name of Imperator Augustus, who named himself after the word augustus, whence English august came. Likewise, both Catholic and catholic derive from a Greek adjective meaning "universal".
Capital letters may be used to differentiate between a set of
objects, and a particular example of that object. For instance in
Astronomical terminology a distinction may be drawn between a moon, any natural satellite, and the Moon, to be specific the natural satellite of Earth. Likewise, Sun with a capital may be used to emphasise that the sun of Earth is under discussion.