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\gls

Synopsis, one of:

\gls{label}
\glspl{label}
\Gls{label}
\Glspl{label}

Refer to a glossary entry. The entries are declared with \newglossaryentry (see \newglossaryentry).

This

\newglossaryentry{N}{%
  name={the natural numbers},
  description={The numbers $0$, $1$, $2$, $\ldots$\@},
  symbol={\ensuremath{\mathbb{N}}},
  }  
  ...
Consider \gls{N}.

gives the output ‘Consider the natural numbers’.

The second command form \glspl{label} produces the plural of name (by default it tries adding an ‘s’). The third form capitalizes the first letter of name, as does the fourth form, which also takes the plural.