Ununoctium and Oganesson refer to the same chemical element, which is noted for its atomic number 118. However, Ununoctium is a temporary placeholder name derived from the element’s atomic number, while Oganesson is the official name adopted by the International Union of Pure and Applied Chemistry (IUPAC).
Ununoctium was primarily used in scientific contexts before the element was confirmed and characterized. Following the element’s discovery in 2002 by a team of Russian and American scientists, it was named Oganesson in 2016 in honor of the physicist Yuri Oganessian, who contributed significantly to the discovery of superheavy elements.
In short, while the names refer to the same element, Oganesson is the recognized and permanent name, whereas Ununoctium is an obsolete term.