Definition of "reducible"

Capable of being reduced.  examples

(mathematics, of a polynomial) Able to be factored into polynomials of lower degree, as x 2 − 1 .  examples

(mathematics, of an integer) Able to be factored into smaller integers; composite.  examples

(topology, of a manifold) Containing a sphere of codimension 1 that is not the boundary of a ball.