您要找的是不是：
n. 发信号；打信号
n. 信号传输法；发信号
In mathematics, a nonempty collection of sets \mathcal{R} is called a σ-ring (pronounced sigma-ring) if it is closed under countable union and relative complementation:
英汉双解词典包含3185865条英汉词条，基本涵盖了全部常用单词的翻译及用法，是英语学习的有利工具。