Ye Old Night Latch
Got an old house with one of these locks?
These "night latches" were added to old doors to utilize the newer style of keys as the old skeleton keys faded out.
While not as strong as the modern deadbolt, you might consider keeping it depending on your situation.
When the night latch was installed, a big chunk of the precious door casing had to be removed, so replacing it with a deadbolt might not be a practical option.
The good news? The cylinder (the round part with the key hole) can be swapped out with a more modern key hole, enabling you to use the same key on the front door as you do the other other entry doors.