WordNet records no synonym and no antonym for frontier. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a wilderness at the edge of a settled area of a country
“the individualism of the frontier in Andrew Jackson's day”
an international boundary or the area (often fortified) immediately inside the boundary
an undeveloped field of study; a topic inviting research and development
“he worked at the frontier of brain science”
Synonyms, antonyms and the broader and narrower relations for frontier come from Princeton University’s WordNet. Full sources and licences.