The horizontal velocity of a projectile can be described in the following ways:

1. Constant Velocity: In the absence of air resistance, the horizontal velocity of a projectile remains constant throughout its flight. This is because there are no horizontal forces acting on the projectile (gravity only acts vertically).

2. Initial Horizontal Velocity (v_x): The horizontal velocity of a projectile is determined by its initial launch conditions. It's often represented as v_x, and it remains constant throughout the flight path.

3. Relationship to Launch Angle (θ): The initial horizontal velocity can be calculated from the initial launch speed (v_o) and launch angle (θ) using the following equation:

v_x = v_o * cos(θ)

4. Components of Motion: The horizontal velocity is one component of the projectile's overall motion. The other component is the vertical velocity, which is affected by gravity and changes throughout the flight.

5. Range and Time of Flight: The horizontal velocity directly affects the range of the projectile (how far it travels horizontally) and, indirectly, its time of flight. A higher horizontal velocity results in a longer range.

Here's a simplified explanation:

Imagine throwing a ball horizontally. The ball will travel in a curved path downwards due to gravity. However, its speed in the horizontal direction will remain the same throughout its flight. This is because nothing is pushing or pulling it sideways.

Important Note: Air resistance will have a minor effect on the horizontal velocity, slowing it down slightly. In most introductory physics scenarios, air resistance is often ignored for simplicity.