Noon; midday — generally the time around 12:00.
昼の12時に病院で待ち合わせをした。
We arranged to meet at the hospital at 12 noon.
今日は昼に会議がある。
There is a meeting at noon today.
正午になると教会の鐘が鳴った。
The church bells rang at noon.
Daytime — the period of the day when it is light outside; can be used adverbially.
昼は子どもたちが外で遊んでいる。
Children play outside during the daytime.
昼間は気温が上がることが多い。
Temperatures often rise during the day.
昼の間に用事を済ませておこう。
Let's take care of errands during the daytime.
Lunch — the meal eaten around midday (often called 'ohiru' in conversation).
今日は昼に何を食べる？
What are you going to eat for lunch today?
昼は会社の近くで弁当を買った。
At lunchtime I bought a bento near the office.
子どもたちは昼に学校で給食を食べる。
The children eat school lunch at midday.