What does vertical bars around a set mean?

[Joseph1739]

I'm trying to figure out what the vertical bars in |A∪B| mean given that A and B are both sets. I tried searching for it, and can't seem to find the meaning of it. Is it supposed to be absolute value of all the elements of the union of sets A and B?

 

[Geofleur]

Putting bars on either side of something usually represents getting the "size" of that something. So, e.g., $| \textbf{v} |$ is the magnitude of a vector. In this case the bars most likely mean the cardinality of the set. For a finite set, that's just the number of elements in the set.