A run-on sentence is a sentence

Sentence (linguistics)

In the field of linguistics, a sentence is an expression in natural language, and often defined to indicate a grammatical unit consisting of one or more words that generally bear minimal syntactic relation to the words that precede or follow it...

 in which two or more independent clauses (i.e., complete sentences) are joined without appropriate punctuation or conjunction. It is generally considered a stylistic error, though it is occasionally used in literature and may be used as a rhetorical device. An example of a run-on is a comma splice

Comma splice

A comma splice is the use of a comma to join two independent clauses. For example:Although acceptable in some languages and compulsory in others, comma splices are usually considered style errors in English.- Prescriptive view :...

, in which two independent clauses are joined with a comma without an accompanying coordinating conjunction.