salt.

"She felt shy with him, yet somehow close, a closeness charged with something she had never felt with Richard. Something suspenseful, that she enjoyed. A little salt, she thought (268)."
It's not till Carol breaks up with Terry that Terry's all-consuming obsession ends, and she seems to come in to her own. It's almost surprising really, how well Terry is able to pick herself up, start her own life, inspired by a piece of music. But perhaps most exciting for me was to finally find a reference to salt, something solid to explain a title that could only otherwise be viscerally understood. Salt is the sexual chemistry that charges situations like the one with Danny. It's something gritty and tasty. Our bodies sweat it, but it burns like hell in an open wound. It's essential. People used to be paid in it. Its at the opposite end of the spectrum from the otherwise opulent trappings that Terry and Carol lavish on themselves. Perhaps that kind of salt comes at a higher price than party dresses, purses and Remy Martin. I would argue that it can't be bought; it's priceless.
But rather than taking her encounter with Danny as some flirtation or contradiction, I would go on to argue that it's a great turning point in Terry's character.
It’s the first time she understands herself as attractive, and what attracts her to others.
She knows exactly what the actress has on her mind when she gets invited to the champagne after-party. And although I think Patricia Highsmith and I might have gone upstairs to find a little salt there (I would turn down champagne only for true love), it is a nice surprise that Terry goes to find Carol, and that we are to assume they live happily ever after.