# What does the $\|$ operation mean in cryptographic notation?

I am studying elliptic curves problems, which also includes study of related protocols such as ECIES. The problem is that I don't understand the notation $\|$. What does this operation mean?

Some stuff is mentioned on Wikipedia's Integration Encryption Scheme article, but this algorithm is better explained in the book Guide to Elliptic Curves. This operation is mentioned in Alice's 3rd step:

$$k_E \| k_M = \operatorname{KDF}(S \| S_1)$$

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.