What does $P(A \triangle B)$ mean? I know its a simple question but its hard to google these symbols. Anyways, anyone care to explain what $P(A \triangle B)$ means? I haven't seen this symbol before and I'm not sure on how to interpret it. 

As in other post, $A\bigtriangleup B$ is the symmetric difference of 2 sets. $P(A\bigtriangleup B)$ is the probability that at least one of $A$ and $B$ but not both happens, i.e. exactly one of $A$ and $B$ happens.
