Commonwealth - A multitude of people who together consent to a sovereign authority, established by contract to have absolute power over them all, for the purpose of providing peace and common defense.
Contract - Also called "covenant" or "social contract," contract is the act of giving up certain natural rights and transferring them to someone else, on the condition that everyone else involved in making the contract also simultaneously gives up their rights. People agreeing to the contract retain only those rights over others that they are content for everyone else to retain over them.
First Principles - The fundamental and irreducible facts of nature that are established by philosophical definition and upon which philosophical arguments may be built. According to Hobbes, first principles are not discovered by observation or experiment but are decided by philosophical debate and social consent.
Law of Nature - A general rule discovered by reason that forbids a person from doing anything destructive to her own life and gives her the right of self-preservation. The laws of nature state that human beings must strive for peace, which is best achieved by contract.
Leviathan - A metaphor for the state, the Leviathan is described as an artificial person whose body is made up of all the bodies of its citizens, who are the literal members of the Leviathan's body. The head of the Leviathan is the sovereign. The Leviathan is constructed through contract by people in the state of nature in order to escape the horrors of this natural condition. The power of the Leviathan protects them from the abuses of one another.
Materialism - The philosophy of materialism states that physical matter and its motion explain all phenomena in the universe and construct the only reality that human beings can experience.
Natural History - The collection of natural objects, organisms, phenomena, and facts gathered by observation.
Natural Man - An inhabitant of the state of nature. Natural men are the main characters of the narrative within Hobbes's text, who escape from their natural condition by making a contract with each other to engineer the Leviathan. Although they are "men," the term also includes women (though the gender significance of this term should not be entirely ignored).
Natural Philosophy - Natural philosophy is the study of nature and the physical universe, and was the intellectual endeavor that eventually led to the historical development of modern science. Natural philosophers such as Francis Bacon and Robert Boyle believed that natural philosophy should derive inductively the workings of nature from natural history. Hobbes believed that natural philosophy should derive deductively the workings of nature from established first principles.
Plenum - Hobbes used the term "plenum" to refer to his conception of the universe; according to this conception, the universe is wholly material in nature, making possible the condition of a vacuum in space. The assumption that the universe is a plenum is an important aspect of Hobbes's materialism.
Sovereign - The person, or group of persons, endowed with sovereignty by the social contract. The sovereign is the head of the Leviathan, the maker of laws, the judge of first principles, the foundation of all knowledge, and the defender of civil peace.
Sovereignty - Supreme authority over a commonwealth. Sovereignty is owed complete obedience by its subjects. Hobbes describes sovereignty as the soul of the Leviathan.
State of Nature - The "natural condition of mankind" is what would exist if there were no government, no civilization, no laws, and no common power to restrain human nature. The state of nature is a "war of all against all," in which human beings constantly seek to destroy each other in an incessant pursuit for power. Life in the state of nature is "nasty, brutish and short."