单扩张是由一个元素的伴随函子生成的域扩张，也是最简单的域扩张，记作Fa。新的域是原域加上新元素而成的最小域。

本原元定理描述了哪些域扩张E/F中的E可以表示为单扩张。

In field theory, a simple extension is a field extension which is generated by the adjunction of a single element. Simple extensions are well understood and can be completely classified.

The primitive element theorem provides a characterization of the finite simple extensions.