An Iso, short for isolation and also known as a Halfback Lead, is a simple run play in American Football which is designed to isolate the fullback on a lead block with a linebacker, giving the halfback an easy 5 yards.
Often seen at the youth and high school levels of football, it takes advantage of mobile quarterbacks and uses the halfback as the lead blocker rather than a fullback.
It is successful against defenses that put a limited number of players in the box.
[2] The play requires a fullback to be successful, and is therefore almost always run out of I-formations.
It helps if the defense is spread across the field, as that makes the combo blocks from the offensive line easier to execute.