The Collaborative International Dictionary of English v.0.48:

Midday \Mid"day`\, n. [AS. middaeg. See Mid, a., and Day.]
   The middle part of the day; noon.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Midday \Mid"day`\, a.
   Of or pertaining to noon; meridional; as, the midday sun.
   [1913 Webster]

WordNet (r) 3.0 (2006):

midday
    n 1: the middle of the day [syn: noon, twelve noon, high
         noon, midday, noonday, noontide]