Refers to the middle of the day, often with a nuance of bright, full daylight. Can be used as a noun or adverbially.
The midday sun is beating down.
Drinking alcohol in broad daylight is crazy.
真昼 refers to broad daylight around midday, while 昼下がり points specifically to the quieter time just after that, in the early afternoon.
真昼 means the middle of the day (broad daylight), the direct opposite of 真夜中, the middle of the night.
Standard kanji spelling; the reading まひる is unambiguous.
Compound of 真 (ま, 'true, just') and 昼 (ひる, 'daytime, noon'), literally 'true noon'.