Here are some examples of how it's used in English words:
* Diameter: The distance across a circle through its center.
* Diagnosis: The identification of a disease or condition through examination.
* Diaphragm: A muscular partition that separates the chest from the abdomen.
* Dialogue: Conversation between two or more people.
* Diarrhea: The frequent passage of loose, watery stools.
The meaning of "dia" can vary depending on the word it's a part of, but it usually relates to a movement or process that involves going through or across something.