What technique is used in the following definition: "RADAR originally stood for 'radio detection and ranging,' but these days it is rarely used as an acronym and instead appears as the noun radar"?

The technique used in the definition provided for RADAR is called an "acronym." An acronym is a form of abbreviation where the initial letters from a series of words are taken and pronounced as a word. In the case of RADAR, it stands for "Radio Detection And Ranging." Over time, the term "radar" has become so common that it is no longer thought of as an acronym and is instead used as a regular noun in the English language. This process, where an acronym becomes a standard word that is no longer recognized as an acronym, is sometimes referred to as lexicalization.