In the book In the Sign of the Beaver, seba is the Indian word for tomorrow.
The word seba first appears at the end of Chapter 6. Matt has foolishly tried to get honey from a honeycomb, and is attacked by bees. To escape his tormentors, he runs into the water, but is caught in the tangle of weeds at the bottom of the pond. Matt is rescued by two Indians who have been quietly observing him; the older one extricates him from the water and carries him back to his cabin, where he and nurses him back to health. Matt is at first delirious from the poison of the bee stings, and when he finally wakes up, he discovers that his rescuers are Saknis, a dignified older man, and a youth about his own age, Attean. The Indians speak some English, and communicate with Matt using a mixture of English and their own language.
When Matt is well enough to get by on his own, the Indians prepare to leave. Matt wants to give them something to show his gratitude, and offers them the only thing he can think of that might be of value - a book. Saknis cannot read, but is impressed that Matt can, and asks him to teach Attean to recognize "the white man's signs". He says that Attean will come for his first lesson seba, or tomorrow (Chapter 6).
Other Indian words that Matt learns from Saknis and Attean include nkweniss, which means grandson, nda, which means no, and piz wat, which means good-for-nothing and is the name that Attean affectionately uses for his dog.