Going all the way across a continent, from one side to the other. It is used most often about big things that cross land: a transcontinental railroad, a transcontinental highway, a transcontinental flight. The picture is of a line drawn right across a huge piece of land. In the United States it has a special weight, because 'the transcontinental railroad' usually means the famous one finished in 1869 that joined the east coast to the west — so for many American speakers the word brings that history with it. It can also describe a company or a journey that stretches across a continent, and sometimes a country that sits in two continents at once (Turkey and Russia are called transcontinental countries, because part of each is in Europe and part in Asia). People sometimes stretch it loosely to mean any very long trip, even one that crosses an ocean, but that use is not careful — for crossing an ocean the right words are 'transatlantic' or 'transpacific'. It is mostly an adjective, but Americans occasionally use it as a noun for a train or flight that makes the crossing.