Which term describes a shortened form of a word, typically consisting of just letters?

The term that describes a shortened form of a word, typically consisting of just letters, is "abbreviation." An abbreviation condenses a word or phrase into a shorter form, often using the initial letters of the words or omitting certain syllables. This allows for easier and faster communication, particularly in written language.

In contrast to other terms, an acronym is a specific type of abbreviation that is formed from the initial letters of a phrase and is usually pronounced as a word itself, such as "NASA" for National Aeronautics and Space Administration. An abbreviated form is a broader term that can refer to various types of shortened words but does not specifically define the concept of using letters. A shortened phrase suggests a more informal or partial shortening of longer sentences rather than the standardized shortening typical of abbreviations. Hence, "abbreviation" precisely captures the meaning of a shortened word consisting of mostly letters.