Caption: A reference frame laid over spacetime (space and time considered jointly).

Spacetime is a common jargon in relativity speak: i.e., the jargon of relativistic physics (combined special relativity (1905) and general relativity (1915)).

The clocks in the image illustrate the time dimension.

Credit/Permission: User:Maschen, 2012 / Public domain.
Image link: Wikipedia: File:Reference frame and observer.svg.
Local file: local link: frame_reference_spacetime.html.
File: Relativity file: frame_reference_spacetime.html.