\newglossaryentry

Synopsis, one of:

\newglossaryentry{label} 
{ 
  name={name}, 
  description={description}, 
  other options, ... 
}

or

\longnewglossaryentry{label} 
{ 
  name={name}, 
  other options ..., 
} 
{description}

Declare a new entry for a glossary. The label must be unique for the document. The settings associated with the label are pairs: key=value.

This puts the blackboard bold symbol for the real numbers ℝ, in the glossary.

\newglossaryentry{R}  
{  
  name={\ensuremath{\mathbb{R}}},  
  description={the real numbers},  
}

Use the second command form if the description spans more than one paragraph.

For a full list of keys see the package documentation on CTAN but here are a few.

name

(Required.) The word, phrase, or symbol that you are defining.

description

(Required.) The description that will appear in the glossary. If this has more than one paragraph then you must use the second command form given in the synopsis.

plural

The plural form of name. Refer to the plural form using \glspl or \Glspl (see \gls).

sort

How to place this entry in the list of entries that the glossary holds.

symbol

A symbol, such as a mathematical symbol, besides the name.