Math-textbook term. A
数域 is a set of numbers closed under +, −, ×, ÷ (excluding division by zero). The standard examples are the rationals
有理数域 (Q) and the reals
实数域 (R).
域 alone means "field" in the algebraic sense — same character is used for "domain" in
函数域 fànghánshùyù.
