Frame of Reference: A frame of reference is a coordinate system used to describe the position and motion of an object. In relativity, the frame of reference of the observer is crucial in determining the observed properties of an object.
Spacetime: Spacetime is the four-dimensional continuum that combines the three dimensions of space and the one dimension of time. Relativity describes the curvature of spacetime and how it is affected by the presence of mass and energy.
Lorentz Transformation: The Lorentz transformation is a set of mathematical equations that describe how measurements of space and time are transformed between different frames of reference moving at constant velocities, as described by special relativity.