Fused sentence
A fused sentence (or run-on sentence) occurs when independent clauses are placed together without punctuation or a conjunction:
She was getting tired she couldn't run any further.
Problem and cure: This sentence is fused because "she was getting tired" and "she couldn't run any further" are both independent clauses, and they are not separated by a period, semi-colon, or a conjunction with a comma.
Many writing instructors frown on fused sentences (and comma splices and sentence fragments) more than the other common usage errors. People sometimes tend to speak rapidly, in what might sound like fused sentences, but written work must account for separate, independent clauses by using one of three standard means:
A yellow dagger of lightning lit up the sky the baby screamed in fear. (fused)
A yellow dagger of lightning lit up the sky. The baby screamed in fear. (clauses separated by period)
A yellow dagger of lightning lit up the sky; the baby screamed in fear. (clauses separated by semi-colon)
A yellow dagger of lightning lit up the sky, and the baby screamed in fear. (clauses separated by comma and conjunction "and")