What Was Meant By Manifest Destiny?

It was during the 1840's that a lot of Americans started to believe that all the land between the Pacific ocean and the Atlantic ocean should be ruled by the United States.  The very notion was termed as the Manifest Destiny.  In the middle of the decade of the 1840's, which was the year 1845, James K. Polk became the president of the United States.  Before becoming president, he had promised to carry out the idea of the Manifest Destiny, further elaborating on his vision that Oregon, Texas and California would be included in the map of the United States.  

Initially the Oregon Country was ruled mutually by Great Britain and the United States.  Later on when the population of Americans exceeded the population of British settlers in Oregon Country, a compromise was signed between the United States and Great Britain that divided the Oregon Country and the part that came under the United States was termed as the Oregon territory.