# ECIES protocol - what does the || operation mean?

I am studying elliptic curves problems, which also includes study of related protocols such as ECIES. A there is a problem I don't understand operation $||$. What this operation mean?

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

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

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