Australian Accounting Standard AASB 1013 defines goodwill as the future benefits from unidentifiable assets.
Wayne Lonergan in the third edition of The Valuation of Businesses, Shares and Other Equity, defines goodwill as:
That part of the difference between the market value of the equity of an entity and the value of its net tangible assets which is not attributable to an identifiable tangible asset.
Goodwill is the unidentifiable intangible asset arising from benefits such as the reputation of a business, its location, its market penetration, good management, good customer and good labour relations.
Goodwill is distinct from identifiable intangible assets such as brand names, licences, patents, trademarks, etc. Goodwill may be purchased (in which case its value can be recognised for accounting purposes) or internally generated (which is generally not brought to account).
A practical example of goodwill calculation is shown here