A neologism is an invented or artificial word.
For example, when scientists invent or discover something new, it requires a name or some other way of referring to it. Sometimes it is named after the inventor or discoverer, like Murphy's Law, and sometimes this leads to a new word entering the language, like 'diesel' for the engine invented by Rudolf Diesel and the fuel it runs on. At other times a new word is used that finds its way into everyday language (or is at least recognised by nearly everyone). A good example of this is the word laser, which is an acronym of Light Amplification through the Stimulated Emission of Radiation and was coined in 1959 by Gordon Gould1 based on the acronym 'maser'2. Similarly, 'radar' stands for 'RAdio Detection And Ranging and was coined by US Navy researchers in 1942. New words in science are often based on existing words, especially Latin and Greek ones, like the term invented by Marie and Pierre Curie for the newly discovered phenomenon 'radioactivity' which shares a Latin root with the name 'radium' for one of the radioactive elements. (And after the potential of radioactivity was fully realised by the wider scientific community, Marie had a element named in her honour, 'Curium'.)