Continuum

Etymology

From Latin continuum, from continuus, “uninterrupted,” from continēre, “to hold together” (con-, “together,” + tenēre, “to hold”). The idea of parts “held together” without gaps is exactly what the word still carries.

noun · /kun-TIN-yoo-um/
A continuous, unbroken sequence in which adjacent parts differ imperceptibly.
What it names
The gapless whole, insisting there are no natural dividing lines — so any boundary we draw on it is a convenience rather than a fact.
Example
Health and illness form a continuum; the line where “tired” becomes “unwell” is one we impose.
Related idea
Both are ranges, but a spectrum tends to have defining endpoints, while a continuum emphasizes the seamless gradation itself.