Sentences that cannot express a complete thought are known as:

Sentences that cannot express a complete thought are referred to as fragmented sentences. A fragment happens when a sentence lacks either a subject or a verb, or does not convey a complete idea, making it unable to stand alone as a complete thought. This is a common issue in writing, where thoughts may be started but not fully developed. Recognizing fragment sentences is important for effective communication, as they can lead to confusion and misunderstanding.

In contrast, the other terms do not accurately describe incomplete sentences in this context. Possessive relates to ownership, run-on refers to sentences improperly connected without suitable punctuation, and partial does not specifically refer to incomplete thoughts in grammatical terms. Understanding the definition of a fragment is crucial for clear and concise writing, especially in technical fields such as engineering where clarity of information is vital.