adapter

As a noun adapter

is adapter (device to allow compatibility).

As an adjective undefined is

lacking a definition or value.

As nouns the difference between adapter and bracket

is that adapter is adapter (device to allow compatibility) while bracket is (senseid)a fixture attached to a wall to hold up a shelf.

As a verb bracket is

to bound on both sides, to surround as enclosing with brackets.

As nouns the difference between reducer and adapter

is that reducer is something that reduces while adapter is adapter (device to allow compatibility).