reducer

nouns

a substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver

reducer, reducing agent, reductant

is derived from

reduce

pipefitting that joins two pipes of different diameter

reducer

is a type of (narrower)

fitting consisting of threaded pieces of pipe for joining pipes together

pipe fitting, pipefitting