Dead End sign
The road ahead terminates with no through connection. You will have to stop, turn around or turn onto a cross street before the end.
Federal designation W14-1 in the MUTCD, the national standard every state signs to. Posted at the entrance to the street itself, before you commit, rather than partway down it, so you get the warning while there is still room to choose a different route.
How to recognise it
This sign is a diamond stood on its point in yellow, the warning colour. Yellow diamonds telling you what the road is about to do. They are advisory rather than mandatory, but ignoring them is how people end up in ditches and how examiners end up failing you.
What the test wants you to know
A dead end still allows turns onto side streets along the way, it is only the road itself that stops, which is what separates it from a full cul de sac with no side access at all. Some states use a T shaped symbol version of this sign instead of the plain text plate, so tests may show either.
The sign people confuse it with
Dead End gets mixed up with No Outlet more than any other pairing in this category. Put them side by side and the difference is obvious, which is exactly why it is worth doing once rather than meeting them cold in the exam.
Practise it
Reading about a sign is not the same as recognising it in half a second with an examiner watching. Every state road signs test on this site shows the sign and asks you to name it, then shows four faces and asks you to pick the right one.