As a noun, the word midday refers to the middle of the day. You should avoid the sun at midday, especially if you have very fair skin.
The noun midday can particularly mean noon. You may prefer to have your heaviest meal midday. The opposite of midday in this case is, of course, midnight. Midday is a compound word combining mid and day. In Old English the term was middæg and German still has a similar word with Mittag. The word midday can also be used as an adjective for midday meal, midday sun.