Amortisation is the intangible’s equivalent of the tangible’s depreciation
It’s intended to amortise an intangible over its estimated useful life (like depreciation does for a tangible asset)
Impairment is a one-off (or two-off, or three-off!) exercise when the directors determine that an asset’s carrying value exceeds its recoverable amount
I suppose that you could equate it with under-depreciation or under-amortisation but its cause by recognising the fact that, because of a change in circumstances, the asset’s carrying value is too great