Formal:
* Sentence fragment: This is the most accurate and formal term, used in grammar and writing instruction.
* Incomplete sentence: This is a less formal way of saying "sentence fragment," but still grammatically accurate.
Informal:
* Phrase: This is less specific than "sentence fragment" and could also apply to grammatically complete groups of words.
* Gibberish: This term is used when the group of words is completely nonsensical or unintelligible.
* Run-on: This term is typically used for two or more sentences joined together without proper punctuation.
Examples:
* Sentence fragment: "Running down the street."
* Incomplete sentence: "Because I was late."
* Phrase: "The big, red ball."
* Gibberish: "Jumble of words with no meaning."
* Run-on: "I went to the store I bought some milk I came home."
The best term to use will depend on the specific situation and your audience.