A continuum is a continuous system or range in which adjacent elements do not vary from each other in any marked degree although the endpoints of the system may be drastically different.
Any continuous whole comprising individual units can be considered a continuum, although a progression of some sort is often considered part of the definition. The set of real numbers (R) is sometimes called the continuum because it is intuitive to think of its elements as corresponding one-to-one with the points on a geometric line.
Examples of continuums:
Sequences of real numbers, ranges of temperatures and the notes of a musical scale are a few common real-world examples of continuums.
The space-time continuum, first described in Albert Einstein's general theory of relativity, is the enmeshed combination of our three perceived physical dimensions, plus time. Time itself is sometimes referred to as a “free-flowing continuum.” Similarly, 3-D space can be considered an unbounded continuum in which exactly three numerical coordinates are necessary to uniquely define the location of any particular point.
See a PBS video explaining the combination of space and time into a single continuum: