I'm reading the 2003 Recurrence Sequences and trying to figure out notation related to S-units. It does not appear earlier in the section (or the book, as far as I can tell) and doesn't appear in the "Notation" section. Full context:
Let $S$ denote the set $S=\{2,3\}$ and write, for any integer $k$, $$|k|_S=|k||k|_2|k|_3$$ for the $S$-norm of $k$. Notice that the $S$-norm of an integer is 1 if and only if the integer comprises powers of 2 and 3 only in its prime factorization: Such an integer is called an &S$-unit.