When Ciri comes to Kaer Morhen in the Netflix series, Vesemir is excited, because he seems to have resigned himself to the decline of the witchers after the destruction of all the potions used to make them. However, Ciri has "Elder blood," which was one of the primary ingredients in the potion to create new witchers. The way the show described the situation, it seemed that Ciri had Elder blood because one of her distant ancestors was an elf. For instance, when discussing Ciri's presumed Elder descent, both Triss and Ciri seem to equate the two.
However, if "Elder" were the same as "elf", it seems hard to see why Vesimir would have been convinced that the situation was hopeless before, and why he would have seen Ciri's arrival as a unique opportunity to turn around the decline of the witchers. While the elves may have been in decline, they were hardly difficult for a witcher to find, what with there being a whole small, nomadic nation of them, and surely some among their number could have been bargained with for a blood sample. People who are mostly human with some elf ancestry seem to be even more common, and I am sure someone like Yennefer would have given Vesemir some blood for the right price.
So what is the distinction, if any, between Elder and elf?