What does the $\|$ operation mean in cryptographic notation?
In terms of algorithms, it usually means concatenate - as in, join two binary (or otherwise defined) strings together (in this order). For wikipedia's cryptography articles that's nearly always the case.

When it is on the left side of an assignment, this means split the string from the right side into these component strings. This makes only sense if the length of most components is already fixed, of course – like here for the encryption and MAC keys.

In terms of programming languages such as C, however, it means logical or. Whilst it is unlikely the authors have confused them, it is something to be aware of.



Different notations may be used for string concatenation, including the single pipe sign $s_1 | s_2$ and the plus sign $s_1 + s_2$, although the double pipe sign as in the question seems prevalent.