In a causality relationship a sufficient condition is any variable which is sufficient to bring about the effect in question. Necessity and sufficiency are terms used to describe a conditional or implicational relationship between statements. An example of sufficient condition is that a growing unemployment rate might be sufficient to cause an increase in the crime rate. A necessary condition is one that must be satisfied for the statement to be true. Typically there are many conditions that are sufficient condition to cause an increase, or a decrease, in crime. The concepts of necessary conditions and sufficient conditions help us understand and explain the different kinds of connections between concepts, and how different states of affairs are related to each other. The concepts of necessary and sufficient conditions play central and vital roles in analytic philosophy.
For example, being an unmarried male is a necessary condition for being a bachelor and being a bachelor is a sufficient condition for being an unmarried male. That a statement is a "necessary and sufficient" condition of another means that the former statement is true if and only if the latter is true. The two statements must be either simultaneously true or simultaneously false.
A condition A is said to be sufficient condition for a condition B, if, and only if, the truth/existence/occurrence, as the case may be, of A guarantees or brings about the truth/existence/occurrence of B.
For example, while air is a necessary condition for human life, it is by no means a sufficient condition, that is, it does not, by itself, alone suffice for human life. While someone may have air to breathe, that person will still die without water, has taken poison, is exposed to extremes of cold or heat, etc.
The foregoing is a complete set of necessary conditions, i.e. the set comprises a set of sufficient condition for x's being square. Frequently the terminology of "individually necessary" and "jointly sufficient" is used. One might say, for example, "each of the members of the foregoing set is individually necessary and, taken all together, they are jointly sufficient for x's being a square."