This article is about a definition in group theory that is standard among the group theory community (or sub-community that dabbles in such things) but is not very basic or common for people outside.
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The virtually operator is a operator from the group property space to itself that takes as input a group property (typically, a subgroup-closed group property) and outputs the property of being a group which has a subgroup of finite index satisfying as an abstract group.
Inportant instances of application of the virtually operator: