1. Through, across, between:
This is the most common meaning. Examples:
* Diameter: The distance across a circle through its center.
* Diagonal: A line segment connecting two non-adjacent vertices of a polygon.
* Dialogue: Conversation between two or more people.
2. Completely, entirely:
This meaning is less common but still occurs. Examples:
* Diametrically opposed: Completely opposite in opinion or direction.
* Diurnal: Occurring daily, during the day.
3. Double, two:
This meaning is less common than the others. Examples:
* Dichromatic: Having two colors.
* Diatomic: Composed of two atoms.
4. In two parts:
This meaning is also less common. Examples:
* Dichotomy: A division into two contrasting parts.
* Diastasis: Separation of normally joined parts.
It's important to consider the context in which "dia-" is used to determine its precise meaning.