This word is properly an adjective, but is commonly called the
   indefinite article. It is used before nouns of the singular number
   only, and signifies one, or any, but somewhat less emphatically. In
   such expressions as "twice an hour," "once an age," a shilling an ounce
   (see 2d A, 2), it has a distributive force, and is equivalent to each,
   every.