"Demi" and "semi" both mean "half" or "partly," and they are often used interchangeably. However, "demi" is generally used for things that are half in size or stature, while "semi" is used for things that are half in completeness or degree.
Here are some examples:
* Demi-god: A being who is half-god and half-human.
* Semi-circle: A shape that is half of a circle.
* Demi-tasse: A small cup for coffee.
* Semi-final: A stage in a competition that is half-way to the final.