The term 'COUNTRY's boundary' can be summed up as the BORDER of a COUNTRY. Borders are typically defined by geographical features such as rivers, mountains, or other natural landmarks, and are often marked by man-made structures such as fences or walls. Borders are important for defining the sovereignty of a nation, and are often the source of conflict between countries. The crossword solution for this term is 'BORDER, ' which is a six-letter word.