A fluid is a substance which flows or is capable of flowing. Fluids are of two types -liquids and gases.
Pressure of a fluid: The force exerted perpendicularly by a fluid at rest on unit area of a surface is called pressure. If the area of the surface is A and the force applied perpendicularly by the fluid is F, then pressure p = F/A.
Buoyancy: A water filled pitcher is easy to move in water but it is not that much easy to move keeping it out of water. The pitcher is felt considerably lighter in immersed condition as upward thrust acts on it.
Therefore, the thrust acting vertically upward is equal to the weight of equal volume of water displaced by the submersed body. For this reason an immersed body apparently losses its weight.