means The single four-dimensional framework that unites the three dimensions of space with time into one continuous fabric, as described by Einstein's relativity.

from A compound of plain English 'space' and 'time' that owes its scientific life to the mathematician Hermann Minkowski, who in the early 20th century reframed his former student Einstein's special relativity geometrically — famously declaring that space and time by themselves would 'fade away into mere shadows,' leaving only their union. The German term was 'Raumzeit'; English took up the literal join.