The symbol for <a href="https://en.wikipedia.org/wiki/domain_of_discourse" class="extiw" title="w:domain of discourse">domain of discourses</a>.
Also in <a href="//commons.wikimedia.org/wiki/Category:Unicode" title="Category:Unicode">Unicode</a>:  <a href="//commons.wikimedia.org/wiki/Category:Unicode_1D400-1D7FF_Mathematical_Alphanumeric_Symbols" title="Category:Unicode 1D400-1D7FF Mathematical Alphanumeric Symbols">U+1D53B</a> (𝔻)
<a href="//commons.wikimedia.org/wiki/User:Manabimasu" title="User:Manabimasu">Manabimasu</a>
2020-09-04
<span class="int-own-work" lang="en">Own work</span>