在数学里，尤其是在序理论里，一个偏序集合 A 的共尾性 cf(A) 是指 A 的共尾子集的势中的最小者。

共尾性的定义依赖于选择公理，因为它利用了所有非空的基数集合都有一个最小成员的事实。偏序集合 A 的共尾性亦可定义成最小的序数 x，使得有着值域共尾于陪域的一个从 x 到 A 的函数。第二个定义不需要选择公理也可以有意义。若假设有选择公理（此条目接下来的部分亦将如此假设），这两种定义将是等价的。

共尾性也可类似地被定义在有向集合上，并且用来广义化网中的子串行概念。

In mathematics, especially in order theory, the cofinality cf(A) of a partially ordered set A is the least of the cardinalities of the cofinal subsets of A.

This definition of cofinality relies on the axiom of choice, as it uses the fact that every non-empty set of cardinal numbers has a least member. The cofinality of a partially ordered set A can alternatively be defined as the least ordinal x such that there is a function from x to A with cofinal image. This second definition makes sense without the axiom of choice. If the axiom of choice is assumed, as will be the case in the rest of this article, then the two definitions are equivalent.