This work has four primary objectives. The first is to define the meaning and characteristics of time (see Section 2). The second is to define a standard unit of matter in terms of light to compliment similar standards for time and length (see Section 2.4). The third is to explain why motion and gravity influence the natural frequency of matter (see Section 5). The fourth is to explain what gravity is and to develop a method for predicting the motion of matter (see Section 7). The result is a surprisingly simple concept based on The Position Definition of Time that defines time, matter, and gravity. In this approach, we combine wave mechanics with gravity by describing the curvature of motion in terms of wave propagation. We demonstrate the validity of this method by comparing it with experimental evidence for objects such as photons, baseballs, and planets. A FORTRAN program is also included for calculating the trajectory of an object or wave of light, the orbit of a planet, and the precession of an orbit.