A continuous range with no steps in it. A spectrum resists the categories laid over it, which is exactly why the word is used of minds.
Latin spectrum (an image, an appearance), from specere (to look, to behold); Newton's use of the word for the band light splits into gave it the modern sense of a range with no steps in it.
A continuous range that resists being cut into boxes. It is used of minds precisely because a mind does not sit at one single point on a line.