The difference between Accidental and Essential

Accidental as an adjective:

Not essential; incidental, secondary.

Accidental as an adjective (philosophy):

Nonessential to something's inherent nature (especially in Aristotelian thought).

Accidental as an adjective (music):

Adjusted by one or two semitones, in temporary departure from the key signature.

Accidental as an adjective:

Occurring sometimes, by chance; occasional.

Accidental as an adjective:

Happening by chance, or unexpectedly; taking place not according to the usual course of things; by accident, unintentional.

Accidental as an adjective (geometry):

Being a double point with two distinct tangent planes in 4-dimensional projective space.

Accidental as a noun:

A property which is not essential; a nonessential; anything happening accidentally.

Accidental as a noun (painting, pluralonly):

Those fortuitous effects produced by luminous rays falling on certain objects so that some parts stand forth in abnormal brightness and other parts are cast into a deep shadow.

Accidental as a noun (music):

A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.

Essential as an adjective:

Necessary.

Essential as an adjective:

Very important; of high importance.

Essential as an adjective (biology):

for survival but not by the organism, thus needing to be ingested

Essential as an adjective:

Being in the basic form; showing its essence.

Essential as an adjective:

Really existing; existent.

Essential as an adjective (of a [[lamination]] of a 3-[[manifold]]):

Such that each complementary region is irreducible, the boundary of each complementary region is incompressible by disks and monogons in the complementary region, and no leaf is a sphere or a torus bounding a solid torus in the manifold.

Essential as an adjective (medicine):

Idiopathic.

Essential as a noun:

A necessary ingredient.

Essential as a noun:

A fundamental ingredient.