A fictional but grounded portrait of one type of ICP customer.
A persona is a character sketch of a real buyer type: what they do, what they care about, what they struggle with, where they hang out. It's not the ICP itself (which is the segment); it's one archetypal person in that segment. Named, backstoried, and memorable enough that your team can reference them.
Personas make your ICP concrete. Instead of marketing to 'VPs of Engineering,' you're marketing to Sam. That specificity shapes copy, product decisions, even pricing. Without personas, marketing stays abstract and bland.