An indefinite article signals that the noun is nonspecific or that other examples of the noun exist (that the noun is not unique). Use an indefinite article (a or an) if a noun is new to the reader or is an arbitrary example of the class that the noun belongs to.
An airplane's shadow is virtually the same size regardless of its altitude.[An signals that airplane is an arbitrary example, that more than one airplane exists.]
--"Take It or Leave It," Valley Comic News
The Greenhouse Effect is a theory in environmental
science which warns of the warming of the globe due to an
increase in carbon dioxide in the atmosphere. [Other theories
exist; the first time an increase is mentioned, it is new to the
See Usage Rules for Articles for further information about how indefinite articles are used in sentences. Also see Choosing between a and an.