In the interest of politics, the word “force” is usually defined, according to the third entry of the Merriam-Webster dictionary, as “violence, compulsion, or constraint exerted upon or against a person or thing.”
(Side note: the first and fourth entries are about the science of physics, the second entry is about the will of a group whether voluntary or mandatory, and the fifth entry is about argument and persuasion which is not the definition of force in regards to politics)
In the philosophy of libertarianism, the initiation of force is immoral and unethical in a free society. The initiation of force is different than self defense. When acting in self defense, one is not initiating force, but reacting to an initial force against the individual.
This doctrine is called the non-aggression principle. It does not exclude self-defense. If a mugger threatened to hurt or kill you unless you give him your money, the mugger is initiating force. If you punch him or otherwise fight back, you are using self-defense, and perfectly acceptable under the non-aggression principle.
The issue some libertarians have is in regards to the semantics of the use of force. Defending oneself is still force. But it is, what objectivist philosopher and author Ayn Rand called, retaliatory force; or what anarcho-capitalist economist and historian Murray Rothbard simply called self-defense.
There are pacifist libertarians who believe the non-aggression principle as generally defined is immoral and impractical in a free society since they believe all force is illegitimate. But they are of a minority within the libertarian community.
Not to mention, the words “aggression,” “coercion,” and “violence” are synonyms of the initiation of force. While synonyms of self defense include “protection,” “safeguard,” and “security.” Source of definitions: the Merriam-Webster dictionary.
As the saying goes in regards to statism, “ideas so good they have to be forced.” In this context “force” means the initiation of force. This statement makes the argument that the statist government makes its ideas mandatory, not voluntary. The counter to that is the libertarian belief in the opposition of initiating force.