run-on sentence / fused sentence
Two sentences jammed with no mark.
A fused sentence is two sentences with no mark between them: The gate jammed the truck waited. A comma splice puts only a comma there. Use a period, a semicolon, or a conjunction.
Examples
UseThe hall was cold. We shut the window.
UseThe hall was cold, and we shut the window.
AvoidThe hall was cold we shut the window.
AvoidThe hall was cold, we shut the window. (A comma splice.)