noun - a word that can serve as the subject or object of a verb

inflection, inflexion - a change in the form of a word (usually by adding a suffix) to indicate a change in its grammatical function

decline, diminution - change toward something smaller or lower

drop-off, falling off, falloff, slump, slack - a noticeable deterioration in performance or quality; "the team went into a slump"; "a gradual slack in output"; "a drop-off in attendance"; "a falloff in quality"

downhill - the downward slope of a hill

incline, slope, side - an elevated geological formation; "he climbed the steep slope"; "the house was built on the side of the mountain"

steep - a steep place (as on a hill)

category, class, family - a collection of things sharing a common attribute; "there are two classes of detergents"