\ensuremath {<math commands>}
In LaTeX 2.09, if you wanted a command to work both in math mode and in text mode, the suggested method was to define something like:
\newcommand{\Gp}{\mbox{$G_p$}}
Unfortunately, the \mbox stops \Gp changing size correctly in (for
instance) subscripts or a fraction.
In LaTeX2e you can define it thus:
\newcommand{\Gp}{\ensuremath{G_p}}
Now \Gp will work correctly in all contexts.
This is because the \ensuremath does nothing, producing simply
G_p, when \Gp is used within math mode; but it ensures that math
mode is entered (and exited) as required when \Gp is used in text
mode.