In a causal relationship a sufficient condition (or variable) is any variable which is sufficient to bring about the effect in question. A necessary condition is one that must be satisfied for the statement to be true.
An example of sufficient condition is that a growing unemployment rate might be sufficient to cause an increase in the crime rate. Obviously many other factors (variables) could also cause the increase. Typically there are many conditions that are sufficient condition to cause an increase, or a decrease, in crime.
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, that is. 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."
Caution: In this example, we have been able, with ease, to list a set of individually necessary conditions that is also sufficient for something's being a square. However, we must not generalize from this simple example and believe that it is usually, or often, a straightforward task to specify sets of conditions that are individually necessary and jointly sufficient. Sometimes it is much easier to specify necessary conditions even though we are unable to specify a set that is jointly sufficient. Other times, the converse is true: for some cases it will be easier to specify sufficient conditions without our being able to specify individually necessary ones.