A perfect fluid is one that is incompressible and frictionless. Let's break down the answer to understand it in detail.

What is a perfect fluid?
A perfect fluid is an idealized model of a fluid that simplifies the analysis of fluid flow problems. It is a theoretical concept used in fluid dynamics to study the behavior of fluids under specific conditions.

Incompressible:
An incompressible fluid is one that does not change its density or volume when subjected to external forces. In other words, the fluid remains constant in density even when subjected to pressure changes. This assumption is often made in fluid dynamics to simplify calculations and analyze the fluid behavior more easily.

Frictionless:
A frictionless fluid is one that does not experience any internal or external friction forces. This means that there is no resistance to fluid flow, and the fluid particles can move without any hindrance. It is an idealized assumption that allows for simpler calculations and analysis.

Why is an ideal fluid considered incompressible and frictionless?
The assumption of an incompressible and frictionless fluid simplifies the mathematical equations used to describe fluid flow. These assumptions allow for the use of fundamental equations such as the continuity equation and Bernoulli's equation, which are derived under these idealized conditions.

When a fluid is considered incompressible, it means that it is assumed to have a constant density throughout its flow. This assumption is valid for liquids, as their density changes very little with pressure variations. For gases, which are compressible, this assumption is not valid, and more complex equations are required to model their behavior.

The assumption of a frictionless fluid is made to simplify the analysis of fluid flow. In reality, all fluids experience some form of internal friction due to viscosity. However, for many fluid flow problems, the effects of viscosity can be neglected, especially when studying large-scale flows or when the fluid has a low viscosity.

Conclusion:
In summary, a perfect fluid is an idealized model of a fluid that is incompressible and frictionless. These assumptions simplify the mathematical equations used to describe fluid flow and allow for easier analysis. However, it is important to note that the concept of a perfect fluid is an idealization and does not fully represent the behavior of real fluids.