For more on the actual text of this landmark case, check out this site . In the simplest light, the primary measure of a monopoly is market share. Such control, while not explicitly illegal, can result in a dampening of competition. Certain characteristics are taken into consideration such as the interchangeability between the two. Exxon and Mobil obviously can be, so they are considered part of the same market.
Courts also consider geographic locations.
Once the market has been defined, the courts must determine the amount of market-share under one specific company's control that is contrary to the law. There is no set amount, but the Justice Department has provided guidelines. The Justice Department used to look solely at market share in terms of a percentage and then determine if that share was excessive. This has been modified somewhat with the introduction of the Herfindahl Index. An index is established for the different sectors and any merger that would exceed the index is immediately blocked.