Unionized is the past form of unionize and, much more often, an adjective. As an adjective it describes a workplace, a group of workers, or a job where a union is in place: "a unionized factory", "unionized nurses", "unionized labour". It says something concrete — that these workers are represented by a union, so their pay and conditions are set by a contract the union negotiated, and they cannot usually be dismissed without a set procedure. That is why the word turns up in comparisons: unionized workers typically earn more than non-unionized workers doing the same job. The opposite is "non-unionized" or simply "non-union". As a verb form it is straightforward — "the staff unionized last year" — and it often appears in the passive, "the plant was unionized in 1998". British spelling is unionised.