This term generally refers to the consolidation of companies. This can be accomplished by mutual agreement or by force. A merger is the combination of two existing companies to create a third. An acquisition is one company purchasing another. When this combination process takes place without consent of one of the parties, it is generally referred to as a takeover.
Resistance to such efforts usually takes one of just a few forms:
1) Government obstruction - various regulations can hinder or even overturn these transactions, often due to concerns regarding anti-trust (market competition) rules; and
2) Internal Governance resistance - many companies will have protections built into their operating procedures, including poison pills and stock purchase restrictions; while most often encountered during hostile takeovers, stakeholders in the companies can often utilize such protections to hinder merger or acquisition efforts.