Fulcrum (noun)

1. A pivot or support on which a lever rests or is supported, which enables it to balance or lift a load.
2. A central point, principle, or thing that plays a crucial role in a situation or system.

Origin:

Late 16th century: from latin, "a prop, support", from fulcire "to prop", from fulcus "a prop".

Examples:

1. The lever was balanced on a fulcrum.
2. The fulcrum of the lever was the pivot point that allowed it to lift the load.
3. The economy was the fulcrum of the political debate.
4. The fulcrum of the argument was the question of fairness.
5. The fulcrum of the system was the computer program that controlled it.