The railroad opened up the frontier of the west during the nineteenth century.
The frontier offered the pioneers new crop land when the old had become depleted.
Is space the final frontier?
WordNet 3.0 Sense Numbers: 1
They have closed the frontier between the countries because of war.
He was picked up crossing the frontier between Greece and Turkey with no papers.
The dean dislikes the frontiers separating the sciences from the humanities. (figurative)
WordNet 3.0 Sense Numbers: 2
She is working on the frontiers of cosmology.
They are pushing the frontiers of our understanding of brain chemistry.
The students didn't know what the frontier of computer technology was twenty years ago.
WordNet 3.0 Sense Numbers: 3