W3C 术语 - axis
axis
中文：轴
解释
The axis is an imaginary alignment line upon which a fraction line is centered. Often, operators as well as characters that can stretch, such as parentheses, brackets, braces, summation signs etc., are centered on the axis, and are symmetric with respect to it.
来源
来自：Mathematical Markup Language (MathML) Version 2.0 (2001-02-21)