What did the 'frontier' mean to Americans?

The word ‘frontier’, basically means a borderland between two countries. In the context of America, the definition of frontier may be considered a bit elastic, that is, having separate meanings in separate contexts. At some references, frontier explains the boundary of the state till the place where the Europeans had reached after the expansion. At other instances, frontier is also considered  the zone of transition where the explorers and colonizers were arriving.  This 'frontier’ was also referred to as the land of opportunities by the explorers. To exploit these lands, expansion of the Europeans continued in the western direction.  When the USA’s continental expansion came to an end in 1892, the concept of frontier was also dissolved.