"昼食" means "lunch," the meal eaten in the middle of the day. It's the standard term for the midday meal. In casual speech, it can be shortened to "昼ごはん."

一日の真ん中に食べる「昼ごはん」を指す言葉です。昼間に食べる食事のことです。日常会話では「昼ごはん」もよく使われます。