What is the exact definition of entdothelial cell fenestrae?

In fact, I want to know that are fenestrae 1.holes which exist between cells or 2.they are holes in a cell( for example, a cell is like a paper we have punched it and the holes are called fenestrae). which one is true?

Fenestrae are basically holes within the cytoplasm of the squamous cell, that allow the transmission of macromolecules, therefore, according to the two options you've provided, your second example is the correct one.