The excess of the purchase price paid for an asset or business over its stated book value. Or, put another way - it's the same as the value of a good relationship that a business has with its customers that is regarded and assessed as an intangible asset.
ABC limited has assets worth $2 Million. XYZ limited buys ABC for $3 Million because of its future profit stream. So $3 Million has been paid for a company (apparently) only worth $2 Million - the $1 Million difference or premium is the Goodwill.