Filter or Filtration is a physical, biological or chemical operation that separates solid matter and fluid from a mixture with a filter medium that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through these medium are described as oversize and the fluid that passes through is called the filtrate. The size of the largest particles that can successfully pass through a filter is called the effective pore size of that. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size, thickness and biological activity). Filtration occurs both in nature and in engineered systems; there are biological, geological, and industrial forms.