The story
From the vertical (as viewed from the opposing endzone) alignment of quarterback, fullback, and running back, particularly when contrasted with the same players' alignments in the T formation.
Etymology from Wiktionary, CC BY-SA 4.0.
I formation
Today it means: A common offensive formation, beginning with the usual five offensive linemen (two offensive tackles, two guards, and a center), the quarterback under center, and two backs in-line behind the quarterback. The base variant adds a tight end to one side of the line and two wide receivers, one at each end of the line. Full definition →