Resolution is considered one of the most important parameters in light microscopy, as the resolving power of your microscope determines the size of the smallest feature within your sample that is optically still detectable. When explaining resolution, the term PIXEL is almost unavoidable. PIXEL is an abbreviation for "picture element" and defines how many single points (elements) your picture consists of.
However, commonly the word PIXEL is used for two different concepts: the elements of your picture on screen, as well as the elements on your camera chip. This can cause no little confusion when trying to explain the concept of resolution to someone who is new to the field of microscopy.
PIXEL size (???which one?? -> camera chip) / magnification = PIXEL size (???which one?? ->image)
We therefore propose the additional use of the word DEXEL, this being an abbreviation for "detector element" and describing the actual physical elements that make up the camera chip. This would lead to a much more straight forward explanation of resolution for a camera based microscope.
Our previous example would now look like that:
DEXEL size / magnifcation = PIXEL size
Now a PIXEL is ALWAYS a picture element, meaning a point source (please see below "A pixel is not a little square"), whereas the word DEXEL covers the physical elements on the sensor of your camera (which may or may not indeed be little squares).
If you find yourself thinking "but... a pixel is a little square...isn't it!?", please read this paper.
Because, frankly, this line of thinking is simply incorrect!