noun
Mathematical term for the set of all possible input values of a function. Contrast with 終域 (codomain) and 定義域 (domain, often used interchangeably).
関数の始域を実数全体とする。
Let the domain of the function be all real numbers.
始域と終域が異なる写像を考える。
Consider a mapping where the domain and codomain differ.
Compound of 始 (beginning, start) and 域 (region, range), likely a calque from Western mathematical terminology.