Difference between 中饭 and 午饭
中饭 and 午饭 both refer to lunch. But as the answer mentioned, they are different in the environment you use it.

午饭 is more formal than 中饭. 午饭 can be used both in spoken and written Chinese. But 中饭 is used in spoken Chinese and mostly in dialect in southern China. As I come from southern Jiangsu(江苏) province, we never say 午饭 in our conversation. 

You can see more here:


午饭 http://www.zdic.net/c/8/da/213242.htm
中饭 http://www.zdic.net/c/d/13d/304410.htm