When you move a heated iron up and down a pair of trousers it causes the trousers to fold and a resulting line is noticeable on the front of the trousers. what do you call that "line"?
That line is called a crease.
a line on cloth or paper where it has been folded or crushed:
He ironed a crease down the front of each trouser leg.
The line from ironing is called a crease.