Isaac Newton was the first to propose that objects could be projected from the earth and placed into orbit around the planet. He suggested that such a projectile would have to be launched horizontally from the top of a very high mountain. He argued that as the launch velocity was increased, the distance that the projectile would fall before hitting the earth would increase, until eventually, the downward fall of the projectile would be just matched by the earth’s surface falling away. At this point the projectile would never hit the ground.
Newton proposed a horizontal launch because he realised that any projectile launched at an angle to the horizontal would attempt to follow an elliptical path and would therefore eventually crash back to earth. He also proposed that if the launch velocity became too great, the projectile would proceed away from the earth and not return. Such a launch velocity became known as escape velocity.
Today we define escape velocity as the velocity at which an object on the surface of a body must be propelled in order not to return to that body under the influence of their mutual gravitational attraction. Newton's theory suggested that the launch must be horizontal but now we know that it can be launched at any direction and still achieve the elliptical orbit around the Earth if launched at sufficiently high velocity.