It's often used in chemistry to refer to the element that comes one place after a known element in the periodic table. For example:
* Eka-aluminum was the predicted name for the element Gallium, which is one place after aluminum in the periodic table.
* Eka-silicon was the predicted name for the element Germanium, which is one place after silicon.
The term "eka-" is derived from the Sanskrit word "eka," which also means "one." This prefix was used by Dmitri Mendeleev to predict the properties of undiscovered elements based on their position in the periodic table.