frontier
n. countablen. the border between two countries, or the edge of a settled area where the wild land begins. It can also mean the limit of what people know about a subject.
n. the border between two countries or the extreme limit of settled land beyond which lies wilderness. Often used metaphorically to describe the furthest limits of knowledge or achievement in a particular field.
The explorers traveled across the frontier into unknown lands.
Scientists are currently working at the frontier of genetic research to find a cure for the disease.
The treaty established a new frontier along the mountain range, effectively ending decades of territorial disputes between the two neighboring nations.
From Middle English frounter, from Old French fronter (whence Modern French frontière), from front.
Often used with the definite article 'the' when referring to the limits of a field of study.