Definition of hairline crack.

A crack is a crack.

Whilst Apple have not (to my knowledge) explicitly “defined” the term - a hairline crack is a single contiguous crack that spontaneously appears across the cover glass. The crack will exhibit no signs of direct impact, or additional cracks that radiate away from a single point. Similarly, the iPad casing will show no signs of impact damage - in particular the edges.

It is notoriously difficult to prove that a crack is the result of a manufacturing or material defect. As such, screen damage is often excluded from warranty claims.

Apple offer a limited one-year warranty - against which you might attempt to make a claim. If making a warranty claim, your should first attempt to make a claim with your retailer - as it is most often the retailer with whom your purchase contract (and therefore your warranty) principally exists. If unsuccessful, you then have the potential backup of making a claim against the manufacturer’s limited warranty.

If you had the foresight of opting to purchase Apple’s AppleCare+ plan, this being an insurance product, you will have the security of being able to make claim with Apple regardless of fault. In such circumstances, a small excess/deductible charge is made - and you will receive a replacement (usually a reconditioned iPad of the same model and specification) iPad.

There is nothing explicit about this in Apple's warranty statements.

A screen glass crack is a screen glass crack.

In most cases with Apple, any screen damage is, usually, considered user damage and is NOT usually covered by the one-year limited manufacturing warranty, but ONLY under user paid, extended, two-year warranty coverage.

To my knowledge, there is no published definition regarding cracks. Whilst there may be Apple-internal documentation to guide assessment, this information is not released more widely.