Deflection
The amount a mat, pad or structural element bends or displaces under load. Some deflection is normal and expected in any material; the concern in ground protection is whether it is small enough, and recovers fully once the load is removed, so that it does not damage the ground or affect the stability of the plant standing on it.
In practice
Deflection is where stiffness shows up on site and strength does not. A mat can be nowhere near failure and still deflect enough to matter, because a mat that sags under the foot delivers its load into a smaller area than its plan size suggests — so the pressure reaching the ground is higher than the calculation assumed. Permanent set after unloading is the sign the material has been worked beyond its elastic range and the item should come out of service.
Where this comes from: Ground protection practice as covered in our CPD-accredited seminar on specifying outrigger mats, under material specification and design considerations.
Where it matters on site
On a crane it also feeds straight into levelness. Outriggers are set level before lifting, and differential deflection between feet under load tilts the machine after that check has been signed off. That is why deflection is worth watching during rigging and during the lift, not only when a mat is inspected on return.