In engineering, a fluid is a substance that can flow and take the shape of its container. It refers to both liquids and gases. Unlike solids, which have a definite shape and volume, fluids are characterized by their ability to flow and undergo deformation under the application of external forces.

Fluids can be classified into two main categories: liquids and gases.

1. Liquids: Liquids are a state of matter with a definite volume but no fixed shape. They are relatively incompressible and tend to maintain their volume under normal conditions. Liquids exhibit viscosity, which is a measure of their resistance to flow. Examples of liquids in engineering applications include water, oil, and hydraulic fluids.

2. Gases: Gases are a state of matter that have neither a definite shape nor a definite volume. They are highly compressible and can expand to fill the entire space available to them. Gases are characterized by their low density and high mobility of molecules. Examples of gases in engineering include air, steam, and various process gases.

Fluids play a crucial role in many engineering disciplines, including mechanical engineering, civil engineering, chemical engineering, and aerospace engineering. Understanding the behavior of fluids is essential for the design and analysis of systems such as pipelines, pumps, turbines, heat exchangers, combustion chambers, and aircraft aerodynamics.

The study of fluid mechanics focuses on analyzing the behavior of fluids, including their motion, forces, pressure, and energy transfer. Fluid mechanics principles and equations, such as the Navier-Stokes equations, are used to model and predict fluid behavior in engineering applications. Computational Fluid Dynamics (CFD) is a specialized field that applies numerical methods and computer simulations to analyze and solve fluid flow problems.

In summary, in engineering, a fluid refers to a substance that can flow and take the shape of its container. Understanding the properties and behavior of fluids is fundamental in various engineering disciplines, enabling the design and analysis of systems that involve fluid flow and interaction.