As nouns the difference between melancholy and nihilism

is that melancholy is black bile, formerly thought to be one of the four "cardinal humours" of animal bodies while nihilism is a philosophical doctrine grounded on the negation of one or more meaningful aspects of life.

As an adjective melancholy

is affected with great sadness or depression.

As adjectives the difference between melancholy and nihilistic

is that melancholy is affected with great sadness or depression while nihilistic is of or relating to nihilism.

As a noun melancholy

is (historical) black bile, formerly thought to be one of the four "cardinal humours" of animal bodies.

As adjectives the difference between gracile and spindly

is that gracile is slender; thin; lean while spindly is characteristic of a spindle; slender and of weak appearance.

As nouns the difference between popularization and adoption

is that popularization is the act of making something popular to the general public while adoption is the act of adopting, or state of being adopted; voluntary acceptance of a child of other parents to be the same as one's own child.

As nouns the difference between cluster and enterprise

is that cluster is cluster (group of galaxies or stars) while enterprise is a company, business, organization, or other purposeful endeavor.

As a verb enterprise is

to undertake an enterprise, or something hazardous or difficult.

In transitive terms the difference between busy and crowd

is that busy is to rush somebody while crowd is to press by solicitation; to urge; to dun; hence, to treat discourteously or unreasonably.

As an adjective busy

is crowded with business or activities; having a great deal going on.

As adjectives the difference between quotidian and regular

is that quotidian is (medicine) recurring every twenty-four hours or (more generally) daily (of symptoms etc) while regular is .

As a noun quotidian

is .

As an adverb regular is

regularly.

As nouns the difference between acclivity and anticline

is that acclivity is (geomorphology) a slope or inclination of the earth, as the side of a hill, considered as ascending'', in opposition to ''declivity'', or ''descending ; an upward slope; ascent while anticline is (geology) a fold with strata sloping downwards on each side.

As adjectives the difference between notional and theorical

is that notional is of, containing, or being a notion; mental or imaginary while theorical is theoretical.

As nouns the difference between mouth and mouths

is that mouth is the opening of a creature through which food is ingested while mouths is plural of lang=en.

As verbs the difference between mouth and mouths

is that mouth is to speak; to utter while mouths is third-person singular of mouth.