What in the what is a Sassenach?
In the first chapter, Claire and Frank have a discussion about the semi-pejorative term Sassenach:
"I distinctly heard the barman at that pub last night refer to us as Sassenachs."
"Well, why not?" said Frank equably. "It only means 'Englishman,' after all, or at worst, 'outlander,' and we're all of that."
"I know what it means. It was the tone I objected to." (1.25-1.27)
When Claire finds herself in the past, she is the ultimate outlander: both English and from another time entirely. However, Jamie calls her Sassenach as a term of affection. Between the two of them, the word represents much about what he loves in Claire—her differences, her otherworldliness, her modernity. Claire is his Sassenach: his time-traveler, his Englishwoman, his wife.
Diana Gabaldon almost titled Outlander as Sassenach, but that doesn't exactly roll off the tongue, does it? Either way, though, the title is a shout-out to our main girl, Claire.