Esso is an international trade name for ExxonMobil and its related companies. Pronounced /ˈɛsoʊ/ (“S-O”), it is derived from the initials of the pre-1911 Standard Oil, and as such became the focus of much litigation and regulatory restriction in the United States. In 1973, it was largely replaced in the U.S. by the Exxon brand, while Esso remained widely used elsewhere. In most of the world, the Esso brand and the Mobil brand are the primary brand names of ExxonMobil, with the Exxon brand name still in use only in the United States alongside Mobil.
In 1911, Standard Oil was broken up into seven regional companies, each with the rights to the brand “Standard” in certain states (plus a number of other companies that had no territorial rights). Standard Oil of New Jersey (“Jersey Standard”) had the rights in that state, plus in Maryland, West Virginia, Virginia, North Carolina, South Carolina, and the District of Columbia. By 1941, it had also acquired the rights in Pennsylvania, Delaware, Arkansas, Tennessee, and Louisiana. In those states, it marketed its products under the brand “Esso”, the phonetic pronunciation of the letters “S” and “O”. It also used the Esso brand in New York and the six New England states, where the Standard Oil Company of New York (Socony – Vacuum, later Socony – Mobil)) had the rights, but did not object to the New Jersey company’s use of the trademark (the two companies did not merge until 1998). However, in the other states, the other Standard Oil companies objected and forced Jersey Standard to use other brand names. In most states the company used the trademark “Enco”, and in a few “Humble”. The other Standard companies likewise were “Standard” or some variant on that in their home states, and another brand name in other states.
This situation was confusing to travelers. In 1973, Standard Oil of New Jersey renamed itself as the Exxon Corporation, and adopted that trademark throughout the country. It however maintained the rights to “Standard” and “Esso” in the states where it held those rights, by a token effort, by selling “Esso Diesel” in those states at stations that sell diesel fuel, thus preventing the trademark from being declared abandoned. It retained the “Esso” brand in Puerto Rico and the United States Virgin Islands until 2008, when it sold its stations there to Total S.A. The ENCO brand name was still used on locations in the Midwest, which were scheduled for abandonment.
Info gleaned from Wikipedia