A frame of reference is an imagined set of rulers rigidly attached to some physical object. These rulers extend infinitely in all three space dimensions. In addition to the rulers, a frame of reference includes an imagined set of infinitely many synchronized clocks attached to all ruler points.

As the object moves along, so do the rulers and clocks move along with the object as one rigid structure. The object's motion may be either translational or rotational or both. The imagined grid of attached rulers and clocks will move likewise.

One refers to such a reference frame as the object's frame of reference. E.g., the imagined set of rulers and clocks imagined as moving along with a moving car would be called the car's frame of reference.

The purpose of a reference frame is to allow the assignment of position and time coordinates to any given event. For example, if lightning strikes a tree along a highway on which a car is traveling, this event can be assigned position and time coordinates in the car's frame of reference as follows.

On the grid of rulers moving with the car, there will be a point P that passes the tree at the moment the lightining strikes. The space coordinates of the lightning event relative to the car's frame of reference are taken to be the coordinates of point P as measured on the car frame's set of rulers. The time coordinate of the lightning event in the car's frame of reference is the reading of the clock mounted at P as P passes the tree. (P passes the tree at the moment lightning strikes the tree.)

Not all frames of reference needed in physics are of the kind described above. The kind described here are used in Newtonian physics. They are also used in the theory of special relativity, if one restrics oneself to non-rotating frames moving with constant velocity (inertial frames). However, the general theory of relativity requires different kinds of reference frames.