Wiktionary
n. A lack of centralised political authority, especially following a conflict, revolution, change of power etc.
Wikipedia
In political science and political history, the term power vacuum is an analogy between a physical vacuum, to the political condition "when someone has lost control of something and no one has replaced them." The situation can occur when a government has no identifiable central power or authority. The physical analogy suggests that in a power vacuum, other forces will tend to "rush in" to fill the vacuum as soon as it is created, perhaps in the form of an armed militia or insurgents, military coup, warlord or dictator.
During or following a civil war there is often a power vacuum of some sort. A power vacuum can also occur following a constitutional crisis in which large portions of the government resign or are removed, creating unclear issues regarding succession to positions of power.
Usage examples of "power vacuum".
Operation Keel, to fill and control that power vacuum using the military and economic incentives.
Small, upstart organizations had sprung up in the power vacuum created by the old Guild's destruction, but they were still too weak to create order among such naturally violent and greed-driven creatures.
Kennedy with his head blown off and Nixon back from the dead, running wild in the power vacuum of Lyndon's hopeless bullshit.
The death of DeVore's earthbound copy - pursued and finally killed by the giant Karr - left a power vacuum in the lower levels, a vacuum soon to be filled by one of DeVore's erstwhile allies, the albino Stefan Lehmann.
But shock had quickly led to the realization that there was a power vacuum.
They discussed computers and the current unstable political situation, with the various Warlords plotting to fill the power vacuum left by the increasingly ineffective government of the Galactic Democratic Republic.
The Republicans, a relatively new party, were moving into a power vacuum at a time when sectional differences -- chiefly over slavery -- had submerged most purely local concerns.
Once Bardel was dead-and stiff-necked Boerab as well, by whatever means possible-the shaman could easily fill the power vacuum he had created.