Definitions for "Necessary condition"
A is a necessary condition for B just in case B can't be true unless A is also true: that is, if B, then A. Generally a placeholder for an unspecified sentence.
a condition is whose absence a given phenomenon will not occur
a condition or property which must belong to the thing in order for it to be what it is
an absolute prerequisite for a desired result to take place
a requirement which must previously or simultaneously exist in order to achieve the goal, an intermediate objective, or a specified effect
Keywords:  circumstance, event, exist, occur
a circumstance that must exist for an event to occur
a minimal requirement, an essential element that must be present in every case