Which modern element was called eka-silicon by Dmitri Mendeleev?

The modern element that Dmitri Mendeleev referred to as eka-silicon is germanium.

In 1871, while developing his periodic table, Mendeleev predicted the existence and properties of several then-unknown elements based on gaps in his system. He used the prefix "eka-" (from the Sanskrit word for "one") to indicate that the hypothetical element stood one place below a known element in the same chemical family. Consequently, he named the element predicted to exist below silicon "eka-silicon." The element was later discovered in 1886 by the German chemist Clemens Winkler, who named it germanium.12