The answer is, it can be either one. What distinguishes a cliff from (for example) a mountain is the angle of the slope: in other words, how quickly the land drops off. Since cliffs generally follow the coastline where they're formed, the edge of the cliff can be either straight or curved; and the face of the cliff can be flat, jagged, or even rounded under.
A synonym for straight is unbent.
Curved.
Humans have a straight curved spine when most animals have a C curved spine.
straight
Straight, curved are hard on your back.
straight
curved
a straight line is mainly x2 a curved line is mainly x3
There can be a combination of the two - a line that is straight for a while and then curved, or a zigzag.
If it is not straight, then it is not a line.
A triangle has to have straight sides
The road curved to the right ahead of them. It was a curved wall, rather than straight.