Neutronium (sometimes shortened to neutrium, also referred to as neutrite) is a possible substance composed entirely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the discovery of the neutron) for the hypothetical "element of atomic number zero" (with zero protons in its nucleus) that he placed at the start of the periodic table (denoted by dash, no element symbol). However, the meaning of the term has changed over time, and since the last half of the 20th century it has been also used to refer to extremely dense substances resembling the neutron-degenerate matter speculated to exist in the cores of neutron stars. Science fiction and popular literature commonly use the term "neutronium" to refer to a very dense phase of matter composed mostly of neutrons.