instantaneous

As an adjective instantaneous

is occurring]], [[arise|arising, or functioning without any delay; happening within an imperceptibly brief period of time.

As an adverb instantly is

(archaic) urgently; with insistence.

As adjectives the difference between instantaneous and tan

is that instantaneous is occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time while tan is of a yellowish-brown.

As a noun tan is

a yellowish-brown colour.

As a verb tan is

to change to a tan colour due to exposure to the sun.

As a numeral tan is

the second cardinal number two, formerly used in Celtic areas, especially Cumbria and parts of Yorkshire, for counting sheep, and stitches in knitting.See Wikipedia article on Yan Tan Tethera

As a proper noun Tan is

an English surname derived from the transliteration of various Asian surnames, particularly originating from China and Vietnam.

As an initialism TAN is

Total Acid Number, the measure of a lubricant's or crude oil's acidity.

As adjectives the difference between instantaneous and transient

is that instantaneous is occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time while transient is passing or disappearing with time; transitory.

As a noun transient is

something which is transient.

As adjectives the difference between instantaneous and direct

is that instantaneous is occurring]], [[arise|arising, or functioning without any delay; happening within an imperceptibly brief period of time while direct is straight, constant, without interruption.

As an adverb direct is

directly.

As a verb direct is

to manage, control, steer.

As adjectives the difference between rapid and instantaneous

is that rapid is very swift or quick while instantaneous is occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time.

As an adverb rapid

is rapidly.

As a noun rapid

is a rough section of a river or stream which is difficult to navigate due to the swift and turbulent motion of the water.

As adjectives the difference between unexpected and instantaneous

is that unexpected is not expected, anticipated or foreseen while instantaneous is occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time.

As adjectives the difference between abrupt and instantaneous

is that abrupt is (obsolete|rare) broken away (from restraint) while instantaneous is occurring]], [[arise|arising, or functioning without any delay; happening within an imperceptibly brief period of time.

As a verb abrupt

is (archaic) to tear off or asunder .

As a noun abrupt

is (poetic) something which is ; an abyss .

As adjectives the difference between unanticipated and instantaneous

is that unanticipated is not anticipated while instantaneous is occurring]], [[arise|arising, or functioning without any delay; happening within an imperceptibly brief period of time.