What is the property that causes the velocity of a body to remain constant unless an external force is applied ?

The property that causes the velocity of a body to remain constant unless an external force is applied is inertia. According to Newton's First Law of Motion, an object at rest remains at rest, and an object in motion remains in motion at a constant velocity (constant speed in a straight line) unless acted upon by a net external or unbalanced force [t1, t3, t7]. This inherent tendency of a body to resist any change in its state of motion is specifically defined as inertia [t1, t10]. While momentum is the product of mass and velocity [t8], and elasticity involves restoring forces after deformation [t9], they do not define this resistance to change in state. Surface tension refers to cohesive forces at a liquid's surface [t5]. Therefore, inertia is the fundamental property described by the law of inertia [t10].

Sources

1. [1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/
2. [2] https://www.khanacademy.org/science/in-in-class11th-physics/in-in-class11th-physics-laws-of-motion/in-in-class-11-newtons-first-law-mass-and-inertia/a/what-is-newtons-first-law
3. [3] http://gpnilokheri.ac.in/GPContent/Final%20Physics%20E-contents.pdf