The solar terminator.

Shame on me, but I had never heard of the solar terminator. Or, to put it a different way, every single time I heard the concept in a context other than SCSI cables I thought it was referring to Terminator, the character in the movies. I was obviously wrong, and I owe to Neal Stephenson that I learned a new tidbit of information today.

They are angling across the terminator -not the robotic assassin of moviedom, but the line between night and day through which our planet incessantly rotates.

(Stephenson: p. 740)