To bring down the size, quantity, quality, value or intensity of something; to diminish, to lower.
To lose weight.
To bring to an inferior rank; to degrade, to demote.
To humble; to conquer; to subdue; to capture.
To bring to an inferior state or condition.
To decrease the liquid content of food by boiling much of its water off.
To add electrons / hydrogen or to remove oxygen.
To produce metal from ore by removing nonmetallic elements in a smelter.
To simplify an equation or formula without changing its value.
To express the solution of a problem in terms of another (known) algorithm.
To convert a syllogism to a clearer or simpler form
To convert to written form. (Usage note: this verb almost always appears as "reduce to writing".)
To perform a reduction; to restore a fracture or dislocation to the correct alignment.
To reform a line or column from (a square).
To strike off the payroll.
To annul by legal means.
To translate (a book, document, etc.).