Goodwill is an accounting concept meaning the value of an entity over and above the value of its assets. The term was originally used in accounting to express the intangible but quantifiable "prudent value" of an ongoing business beyond its assets, resulting perhaps because the reputation the firm enjoyed with its clients.
Modern meaning
For example, a software company may have net assets (consisting primarily of miscellaneous equipment, and assuming no debt) valued at $1 million, but the company's overall value (including brand, customers, intellectual capital) is valued at $10 million. Anybody buying that company would book $10 million in total assets acquired, comprising $1 million physical assets, and $9 million in goodwill. In a private company, goodwill has no predetermined value prior to the acquisition; its magnitude depends on the two other variables by definition. A publicly traded company, by contrast, is subject to a constant process of market valuation, so goodwill will always be apparent.