Compact, consolidate, unify, concentrate are comparable when meaning to bring or gather together the parts, particles, elements, or units of a thing so as to form a close mass or an integral whole.
Compact stresses the process more than the effect. It usually suggests a packing or pressing together of many things so as to form a closely arranged mass or a dense substance and may be used in reference both to material and immaterial things; sometimes it carries so strong an implication of solid formation or construction that it fundamentally means to build firmly or to strengthen.
Consolidate implies a merging or uniting, often in an exceedingly close union, of previously distinct but usually homogeneous or complementary things. The term may take as its object such a whole as a nation, a people, or an empire or as a substance or material and may imply a process which promotes the binding together of the parts, elements, or individuals so that solidarity or solidity is achieved.
But consolidate can also take as its objects the units (as parts, elements, individuals, or groups) which have been brought together in close union.
Unify implies a union of heterogeneous or homogeneous parts, elements, or individuals that results in the making or producing of a thing that has oneness and integrity and that stands by itself as a thing apart; the term does not, however, carry as strong an implication of solidarity as does consolidate but, on the other hand, it places stress on the integration of parts so that each does its appointed work or serves its own purpose to the benefit not only of itself but of the whole; thus, a dramatist unifies (not consolidates) the play he composes; after a civil war, the task of the government is to unify (rather than consolidate) a nation; the imagination of a great poet unifies a mass of images and impressions; the Homeric poems may have been originally a collection of narrative poems, but it seems likely that one person unified them.
Concentrate usually carries the implication of bringing together a number of things that are scattered or diffused and of massing them around a point or center.
Concentrate may be extended to imply the fixing of the mind or attention on one thing so that all distracting objects or thoughts are eliminated.
A similar implication of eliminating whatever weakens, dilutes, or adulterates is found in technical use; thus, the chemist concentrates a solution by evaporating the solvent; a miner concentrates ores (i. e., separates the base from the precious metals) by a machine or by washing.