A unified field theory unifies fields (obviously
). Eg, there is a theory which unifies electromagnetism with the "weak nuclear force" - that was done about 20 years ago. The "Holy Grail" of modern physics is to unify "fields" (that is, things like gravity, electromagnetism, etc) with quantum mechanics. You've probably heard of things like "string theory" - that is an attempt to do that, but without any real success thus far. A theory which succeeded in unifying all the "fundamental forces" with QM would be a "Grand Unified Theory", or GUT for short.