also: isolation · lead · lead draw (different play)
A run that deliberately leaves one linebacker unblocked so a fullback can block him one-on-one. Two names, one play, and the name tells you what it is doing.
The offensive line blocks everybody except one linebacker, and a fullback or lead back is sent through the hole to block that man by himself. The runner follows immediately behind him.
The name is the design: the linebacker is isolated on purpose, so the play is a deliberate one-on-one between him and a lead blocker. That makes it the most honest play in football and the easiest to teach, because there is no misdirection and nothing to read. It also makes it entirely dependent on winning a specific collision, which is why it works in some programs and not others with identical diagrams.
isolation, leadduo, dive, drawIso and lead are two names for the same play in most rooms. Where a staff separates them, lead usually names any run with a blocker in front and iso names this specific one-on-one design, so every iso is a lead and not every lead is an iso.
Common at youth level for the reason it is simple. It is also the run most likely to work with a young back, because it needs no read at all: he goes where the fullback goes.