The difference between Flat and Slight

Flat as an adjective:

Having no variations in height.

Flat as an adjective (music, voice):

Without variations in pitch.

Flat as an adjective (slang):

Describing certain features, usually the breasts and/or buttocks, that are extremely small or not visible at all.

Flat as an adjective (music, note):

Lowered by one semitone.

Flat as an adjective (music):

Of a note or voice, lower in pitch than it should be.

Flat as an adjective (of a tire or other inflated object):

Deflated, especially because of a puncture.

Flat as an adjective:

Uninteresting.

Flat as an adjective:

Of a carbonated drink, with all or most of its carbon dioxide having come out of solution so that the drink no longer fizzes or contains any bubbles.

Flat as an adjective (wine):

Lacking acidity without being sweet.

Flat as an adjective (of a battery):

Unable to emit power; dead.

Flat as an adjective (juggling, of a throw):

Without spin; spinless.

Flat as an adjective (figurative):

Lacking liveliness or action; depressed; dull and boring.

Flat as an adjective:

Absolute; downright; peremptory.

Flat as an adjective (phonetics, dated, of a [[consonant]]):

sonant; vocal, as distinguished from a sharp (non-sonant) consonant

Flat as an adjective (grammar):

Not having an inflectional ending or sign, such as a noun used as an adjective, or an adjective as an adverb, without the addition of a formative suffix; or an infinitive without the sign "to".

Flat as an adjective (golf, of a [[golf club]]):

Having a head at a very obtuse angle to the shaft.

Flat as an adjective (horticulture, of certain [[fruit]]s):

Flattening at the ends.

Flat as an adjective (authorship, figuratively, esp. of a character):

Lacking in depth, substance, or believability; underdeveloped; one-dimensional.

Flat as an adverb:

So as to be flat.

Flat as an adverb:

Bluntly.

Flat as an adverb:

Not exceeding.

Flat as an adverb:

Completely.

Flat as an adverb:

Directly; flatly.

Flat as an adverb (finance, slang):

Without allowance for accrued interest.

Flat as a noun:

An area of level ground.

Flat as a noun (music):

A note played a semitone lower than a natural, denoted by the symbol ♭ sign placed after the letter representing the note (e.g., B♭) or in front of the note symbol (e.g. ♭♪).

Flat as a noun (informal, automotive):

A flat tyre/tire.

Flat as a noun (in the plural):

A type of ladies' shoes with very low heels.

Flat as a noun (in the plural):

A type of flat-soled running shoe without spikes.

Flat as a noun (painting):

A thin, broad brush used in oil and watercolor/watercolour painting.

Flat as a noun (swordfighting):

The flat part of something: The flat side of a blade, as opposed to the sharp edge. The palm of the hand, with the adjacent part of the fingers.

Flat as a noun:

A wide, shallow container.

Flat as a noun (mail):

A large mail piece measuring at least 8 1/2 by 11 inches, such as catalogs, magazines, and unfolded paper enclosed in large envelopes.

Flat as a noun (geometry):

A subset of n-dimensional space that is congruent to a Euclidean space of lower dimension.

Flat as a noun:

A flat-bottomed boat, without keel, and of small draught.

Flat as a noun:

A straw hat, broad-brimmed and low-crowned.

Flat as a noun (rail, US):

A railroad car without a roof, and whose body is a platform without sides; a platform car or flatcar.

Flat as a noun:

A platform on a wheel, upon which emblematic designs etc. are carried in processions.

Flat as a noun (mining):

A horizontal vein or ore deposit auxiliary to a main vein; also, any horizontal portion of a vein not elsewhere horizontal.

Flat as a noun (obsolete):

A dull fellow; a simpleton.

Flat as a noun (technical, theatre):

A rectangular wooden structure covered with masonite, lauan, or muslin that depicts a building or other part of a scene, also called backcloth and backdrop.

Flat as a verb (poker slang):

To make a flat call; to call without raising.

Flat as a verb (intransitive):

To become flat or flattened; to sink or fall to an even surface.

Flat as a verb (intransitive, music, colloquial):

To fall from the pitch.

Flat as a verb (transitive, music):

To depress in tone, as a musical note; especially, to lower in pitch by half a tone.

Flat as a verb (transitive, dated):

To make flat; to flatten; to level.

Flat as a verb (transitive, dated):

To render dull, insipid, or spiritless; to depress.

Flat as a noun (chiefly, British, New England, New Zealand, and, Australian, archaic, _, elsewhere):

An apartment, usually on one level and usually consisting of more than one room.

Slight as an adjective:

Small in amount, gentle, or weak; inconsiderable; unimportant; insignificant; not severe.

Slight as an adjective:

Not stout or heavy; slender.

Slight as an adjective (regional):

Even, smooth or level; still (of the sea).

Slight as an adjective (obsolete):

Foolish; silly; weak in intellect.

Slight as an adjective (regional, obsolete):

Bad, of poor quality (as goods).

Slight as an adjective (dated):

Slighting; treating with disdain.

Slight as a verb (transitive):

To treat as slight or not worthy of attention; to make light of.

Slight as a verb (transitive):

To give lesser weight or importance to.

Slight as a verb (transitive):

To treat with disdain or neglect, usually out of prejudice, hatred, or jealousy; to ignore disrespectfully.

Slight as a verb (intransitive):

To act negligently or carelessly.

Slight as a verb (transitive, military, of a fortification):

To render no longer defensible by full or partial demolition.

Slight as a verb (transitive):

To make even or level.

Slight as a verb (transitive):

To throw heedlessly.

Slight as a noun:

The act of slighting; a deliberate act of neglect or discourtesy.

Slight as a noun (obsolete):

Sleight.