means A continuous range of values, qualities, or wavelengths between two extremes, with no sharp dividing lines along the way.

from Straight from Latin spectrum, 'an appearance' or 'apparition' — literally something seen — from specere, 'to look,' the same root behind inspect, spectacle, and spectator. It first carried a ghostly tinge ('specter' is its cousin) before Newton borrowed it in the 17th century for the band of colors a prism conjures from white light, and from that rainbow it spread outward to mean any graded range at all.