The term 'frontier, once' can be represented by the crossword solution 'WEST'. The term 'WEST' is used to describe the direction that is opposite of east. Historically, the term 'frontier, once' was used to refer to the area of land in the United States that was located on the western side of the Mississippi River. This area was largely unexplored and was considered to be the edge of the known world. It was a place of great opportunity and danger, and it was often referred to as the 'Wild WEST'.