Seigan in Kenjutsu refers to middle sword position standing in migi (right) kamae similar to Chudan referred to in Aikido.
This will vary greatly from ryu to ryu. While it is true that in general, especially kendo circles, seigan is known as a sort of "high chudan", there are many ryu which use the term with different purposes and very different physical manifestations. In my own ryu "seigan" is used to define a very different concept than the "pointed at the eyes" descriptions most kendo types will give you. Also while our version of the kamae seigan does involve a certain degree of "pointing at the eyes" (although this is not the primary purpose of it), the actual kamae is very different to the "high chudan" most think of when discussing "seigan".
So simply put, in most modern circles seigan equals a high chudan with the kissaki pointed at the enemy's eyes, but when you start getting into the details of different ryu, that understanding may not apply.
For what it's worth,