Often when doing a substitution for an exact word, say user to admin, I will include spaces on either end of the regex pattern to avoid unintentional replacements. For example, I may use something like

:%s/ user / admin /

in order to avoid a substitution like username to adminname.

In this case, the spaces can be replaced with zero-width regex characters that match the beginning and end of a word. These are \< and \>, respectively. Utilizing these, the previous substitution can be achieved with

:%s/\<user\>/admin/

See :h /\< and :h /\> for more details.