run-on sentence
Two independent clauses jammed together without the punctuation they need.
A run-on sentence joins two independent clauses without correct punctuation between them. A first-degree run-on has nothing at all between the clauses (most handbooks call this a fused sentence); a second-degree run-on has a coordinating conjunction but is missing the comma that belongs in front of it.
Examples
I left she stayed. (first-degree, a fused sentence)
The flight was delayed for hours so we missed the meeting. (second-degree, missing the comma)