(transitive) To bring down the size, quantity, quality, value or intensity of something; to diminish, to lower.

(intransitive) To lose weight.

(transitive) To bring to an inferior rank; to degrade, to demote.

(transitive) To humble; to conquer; to subdue; to capture.

(transitive) To bring to an inferior state or condition.

(transitive, cooking) To decrease the liquid content of food by boiling much of its water off.

(transitive, chemistry) To add electrons / hydrogen or to remove oxygen.

(transitive, metallurgy) To produce metal from ore by removing nonmetallic elements in a smelter.

(transitive, mathematics) To simplify an equation or formula without changing its value.

(transitive, computer science) To express the solution of a problem in terms of another (known) algorithm.

(transitive, logic) To convert a syllogism to a clearer or simpler form.

(transitive, law) To convert to written form. (Usage note: this verb almost always appears as "reduce to writing".)

(transitive, medicine) To perform a reduction; to restore a fracture or dislocation to the correct alignment.

(transitive, military) To reform a line or column from (a square).

(transitive, military) To strike off the payroll.

(transitive, Scotland, law) To annul by legal means.

(transitive, obsolete) To translate (a book, document, etc.).