This term refers to the manner in which the possible vertical range of human hearing is characterised through the occupancy and evolution of the frequency content of sound.
The concept is key to Spectromorphological theory, and the composer and writer Denis Smalley introduces extremely detailed descriptive termninology in "Spectromorphology: Explaining Sound-shapes". Organised Sound Vol. 2, No. 2. Cambridge: Cambridge University Press, 1997.
It should be noted that it is difficult to separate this concept from the perception of pitch, harmonicity, timbre, sound quality and motion.