IPA: /nɒnˈsɛntəns/
KK: /nɑnˈsɛntəns/
A phrase or group of words that does not form a complete sentence and lacks grammatical structure.
The phrase 'running fast' is a nonsentence because it does not express a complete thought.
Nonsentence is formed from "non-" (meaning not) and "sentence" (from Latin "sententia", meaning opinion or thought). The word "nonsentence" refers to something that is not a complete or coherent thought or statement.
Think of 'non-' meaning not, and 'sentence' referring to a complete thought — so a nonsentence is something that does not express a complete thought.