1. The problem asks: What is a fluid column? 2. A fluid column is a vertical section of fluid contained between two surfaces, typically between the bottom and the top of a fluid body. 3. It is important in fluid mechanics because the pressure at any point in the fluid column depends on the height of the fluid above that point. 4. The pressure at depth $h$ in a fluid column of density $\rho$ under gravity $g$ is given by the hydrostatic pressure formula: $$p = p_0 + \rho g h$$ where $p_0$ is the pressure at the surface of the fluid. 5. This concept helps explain phenomena like why pressure increases with depth in oceans or why fluid levels balance in connected vessels. Final answer: A fluid column is a vertical section of fluid whose pressure at any depth depends on the height of fluid above it, described by $p = p_0 + \rho g h$.