Eka-osmium was a temporary name used to refer to the element that goes under osmium in the periodic table. The name "eka" was used in the same way as in Mendeleev's predicted elements. During the first half of the 20th century, eka-osmium referred to plutonium, because the actinide concept, which postulates the actinides form an inner transition series similar to the lanthanides, had not been proposed yet. Once the actinide concept became widely accepted, eka-osmium started to refer to element 108, now called Hassium, which was discovered in 1984.