The difference between Decline and Reduction

When used as nouns, decline means downward movement, fall, whereas reduction means the act, process, or result of reducing.

Decline is also verb with the meaning: to move downwards, to fall, to drop.

check bellow for the other definitions of Decline and Reduction

Decline as a verb (intransitive):

To move downwards, to fall, to drop.

Examples:

"The dollar has declined rapidly since 2001."
Decline as a verb (intransitive):

To become weaker or worse.

Examples:

"My health declined in winter."
Decline as a verb (transitive):

To bend downward; to bring down; to depress; to cause to bend, or fall.
Decline as a verb (transitive):

To cause to decrease or diminish.
Decline as a verb:

To turn or bend aside; to deviate; to stray; to withdraw.

Examples:

"a line that declines from straightness"

"conduct that declines from sound morals"
Decline as a verb (transitive):

To refuse, forbear.

Examples:

"On reflection I think I will decline your generous offer."
Decline as a verb (transitive, grammar, usually of substantives, adjectives and pronouns):

To inflect for case, number and sometimes gender.
Decline as a verb (by extension):

To run through from first to last; to repeat like a schoolboy declining a noun.

Examples:

"rfquotek Shakespeare"
Decline as a verb (American football, Canadian football):

To reject a penalty against the opposing team, usually because the result of accepting it would benefit the non-penalized team less than the preceding play.

Reduction as a noun:

The act, process, or result of reducing.
Reduction as a noun:

The amount or rate by which something is reduced, e.g. in price.

Examples:

"A 5% reduction in robberies"
Reduction as a noun (chemistry):

A reaction in which electrons are gained and valence is reduced; often by the removal of oxygen or the addition of hydrogen.
Reduction as a noun (cooking):

The process of rapidly boiling a sauce to concentrate it.
Reduction as a noun (mathematics):

The rewriting of an expression into a simpler form.
Reduction as a noun (computability theory):

a transformation of one problem into another problem, such as mapping reduction or polynomial reduction.
Reduction as a noun (music):

An arrangement for a far smaller number of parties, e.g. a keyboard solo based on a full opera.
Reduction as a noun (philosophy, phenomenology):

A philosophical procedure intended to reveal the objects of consciousness as pure phenomena. (See phenomenological reduction.)
Reduction as a noun (medicine):

A medical procedure to restore a fracture or dislocation to the correct alignment.