Nuclear pore, a large protein complex that penetrates the nuclear envelope in eukaryotic cells
Ion channel pore; The ion-selective opening in the membrane of a eukaryotic cell formed by members of the ion channel family of proteins
A water-selective opening (water channel) in the membrane of a eukaryotic cell formed by assemblies of the protein aquaporin
In the physical sciences:
One of many small openings in a solid substance of any kind that contribute to the substance's porosity (typical usage in earth sciences, materials science and construction)
A small defect in the crystal structure that may arise during sintering to form solids from powders, including ceramics