The Collaborative International Dictionary of English v.0.48:

out-of-doors \out-of-doors\ n.
   Any location outside of any building, where the air is
   unconfined; the open air.

   Syn: outdoors, air, open air, open.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

out-of-doors
    n 1: where the air is unconfined; "he wanted to get outdoors a
         little"; "the concert was held in the open air"; "camping
         in the open" [syn: outdoors, out-of-doors, open air,
         open]

The Devil's Dictionary (1881-1906):

OUT-OF-DOORS, n.  That part of one's environment upon which no
government has been able to collect taxes.  Chiefly useful to inspire
poets.

    I climbed to the top of a mountain one day
        To see the sun setting in glory,
    And I thought, as I looked at his vanishing ray,
        Of a perfectly splendid story.

    'Twas about an old man and the ass he bestrode
        Till the strength of the beast was o'ertested;
    Then the man would carry him miles on the road
        Till Neddy was pretty well rested.

    The moon rising solemnly over the crest
        Of the hills to the east of my station
    Displayed her broad disk to the darkening west
        Like a visible new creation.

    And I thought of a joke (and I laughed till I cried)
        Of an idle young woman who tarried
    About a church-door for a look at the bride,
        Although 'twas herself that was married.

    To poets all Nature is pregnant with grand
        Ideas -- with thought and emotion.
    I pity the dunces who don't understand
        The speech of earth, heaven and ocean.
                                                       Stromboli Smith