Alliance is a mutually agree to commercial collaboration between two to more organizations where they agree to co-operate in the operation of a business activity. They remain independent entities however.
Merger/amalgamation means combination of two or more companies into a single company, where one company survives with its name or a combined new name, and other loses its existence.
Consolidation is the combination of two existing companies into a new company where old companies lose their existence and a new one is created either with a different name or same name.
Acquisition or takeover of a company refers to the acquiring of a controlling stake in the ownership of a company by another entity. This is done by buying the share capital of another company.