A comma splice results from placing only a comma between two independent clauses, as in this tweet:

One thing is certain, girls everywhere need education.

A related construction is a fused, or run-on, sentence, which results from joining two independent clauses with no punctuation or connecting word between them.

As a fused sentence, the tweet above would read:

One thing is certain girls everywhere need education.

Using comma splices is increasingly common in writing that aims for a casual, informal feel, but comma splices and fused sentences in academic writing are likely to draw an instructor’s criticism. If you use comma splices and fused sentences in formal writing, be sure your audience can tell that you are doing so for a special effect.