A fused sentence — often called a run-on sentence — happens when two complete, independent sentences get jammed together with no punctuation or connecting word between them at all.
These two get confused because they're close cousins. A comma splice joins two sentences with just a comma — at least there's some punctuation. A fused sentence has none at all, which makes it feel even more like the writer lost track of where one thought ended and the next began.
They're most common when you're writing quickly and thinking faster than you're typing — two related thoughts arrive close together and land on the page without the punctuation your brain would naturally insert if you slowed down. Reading your own writing back out loud is one of the more reliable ways to catch these, because you'll naturally pause between the two ideas even when the page shows no pause at all.
A sentence can be long and still be grammatically fine, as long as it's properly connected with conjunctions, subordination, or punctuation. "Fused" specifically means two independent clauses with nothing joining them — length isn't the issue, missing structure is.
Fused sentences are easy to miss on your own re-read because you already know where the mental "pause" belongs, even if the page doesn't show it. Proofread Bot reads the sentence structure directly and flags fused sentences along with a suggested fix.
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