Commensurable subgroups

Symbol-free definition
Two subgroups of a group are said to be commensurable if their intersection has finite index in both of them.

Definition with symbols
Two subgroups $$H_1$$ and $$H_2$$ of a group $$G$$ are said to be commensurable if both $$[H_1:H_1\cap H_2]$$ and $$[H_2:H_1 \cap H_2]$$ are finite.