What is the difference between the following concepts: motion, speed, velocity, acceleration, inertial motion, relative motion, relative velocity, displacement, and relative displacement?
Motion is “the passage of a body from one place to another.” (Webster’s Dictionary, p. 886)
Speed is “the rate of movement or motion.” (Id., p. 1288) Specifically, ‘speed’ is the distance that a body moves from one position to another position, divided by the time elapsed: S = d/t.
Velocity is “the rate of change of position [of a body] in relation to time…in a particular direction.” (Id., p. 1479)
Uniform rectilinear velocity is where the speed of a body is constant in the same rectilinear direction.
Acceleration occurs when the speed or the direction of a body’s velocity is changed.
Inertial motion (in classical physics) is where the uniform rectilinear velocity of a body does not result from an apparent force.
Relative motion is the motion of one body relative to the motion of another body.
Relative velocity is the rate of change of position of one body (or thing) in a particular direction as compared to the rate of change of position of another body (or thing) in its particular direction.
Displacement is “the difference [in distance] between a later position of a thing and its original position.” (Id., p. 396)
Relative displacement is the distance of displacement of one body or thing relative to another body or thing.
 In Special Relativity inertial motion may result from a force.