A Continuum

1. A continuous series or whole, no part of which is noticeably different from its adjacent parts, although the ends or extremes of it are very different from each other.
2. A continuous extent. en.wiktionary.org/wiki/cepstrum#English
3. (mathematics) The nondenumerable set of real numbers; more generally, any compact connected metric space. Continuum (set theory) - Wikipedia Linear continuum - Wikipedia Continuum (topology) - Wikipedia
4. (music) A touch-sensitive strip, similar to an electronic standard musical keyboard, except that the note steps are ^1^⁄₁₀₀ of a semitone, and so are not separately marked.

Spectral Lines vs Continuum:

In plot of I(λ) vs. λ for a star we find that the graph varies smoothly over most wavelengths. There are some wavelength ranges at which there is a sharp increase or decrease in I(λ) over a very narrow wavelength range. These sharp increases and decreases are called spectral lines. In this chapter, we will be concerned with the smooth or continuous part of the spectrum. This is called the continuum.