The \boldmath declaration switches
to a bold math italic font; this causes letters, numbers, and
most symbols used in math mode to be set in a bold type.
The \unboldmath declaration switches
back to the normal math italic font.
Note, these commands should not be used in math mode. Thus the scope of the declaration must include the math mode expressions to which you want it to apply.
If you
want only part of a formula to be bold an
\mbox command is useful:
\( Regular formula +
\mbox{\boldmath$ Bold part$} = 0 \)
Both \boldmath and \unboldmath commands are
fragile.