The British Mandate

In 1920, the Council of the League of Nations appointed Britain as the Mandatory entrusted with the administration of the Land of Israel. The borders of the land, as a separate country, were defined for the first time in many centuries. Until then, under the Ottoman Empire, the land's boundaries had not been defined because it was part of other large Ottoman districts like the district of Damascus and was not a distinct political unit. The term name "Palestine" that was chosen for this Mandate was based on the term name "Palestina" that was given to the country by the Roman Empire in the second century CE.

The territory of the British Mandate included land on both sides of the Jordan River, encompassing the present-day countries of Israel and Jordan. About 77% of this Mandate was east of the river Jordan River, and in 1921, Great Britain created there a separate administrative entity called Transjordan. The changed mandate took effect in 1923.

Israel Ministry of Foreign Affairs

http://www.mfa.gov.il/MFA/AboutIsrael/Maps/Pages/The%20League%20of%20Nations%20Mandate%20for%20Palestine%20-%201920.aspx